When wifi is enabled, and the phone is not connected to a wifi network, it scans for known networks. It is due to networks send out their SSID and if this SSID is in the list of known networks on the phone, the phone tries to connect.
Users say that the phone actively tries to connect to known networks even if they are not available.
This could be due to ability to have hidden networks, i.e. WiFi networks that don't provide their SSID. The phone would never be able to automatically connect to such networks.
There is no possible way to do so in stock Android, nor such functionality is available in custom ROMs. It should be possible user change the source code and compile a ROM yourself,
Most networks send out their SSID. If wifi is enabled and user haven't disabled scanning then phone will listen for those beacons and try to match the SSID in the beacon against user phone's list of SSID's. Depending on your settings it will also use the beacon to determine your current location. Possible solution could be to purge hidden networks from list of known networks.
On some systems user can go to the advanced configuration of an individual network and tell it to not connect automatically, and then it won't probe for it, but Android does not have this feature, so user need to purge and avoid using hidden networks.
Following information can be found when the device is near a Wifi Access Point even though the device isn't connected:
ESSID, Signal strength, Bit rate, frequency channel, Encryption, RTS, Fragmentation threshold. To manipulate the basic wireless parameters, there is iwconfig command.
It is not possible to connect to multiple WiFi networks simultaneously as Android does not support load balancing, it uses one active Internet/network connection for it's primary route.
User complaints that android phone has been losing it's WiFi signal momentarily, typically after few minute intervals. This causes an interruption of several seconds while the WiFi connection is re-established and typically fails any kind of download/streaming that is happening, makes web sites "unreachable" and generally makes the phone unusable as a data device due to the frequency.
The signal remains down for about a second, but the phone takes a few more seconds to reconnect to the router.
This happens regardless of proximity to the router, which has a very strong signal. It also happens whether the phone is plugged in or on battery.
Some specific options can be tried as Changing channels, turning off the router's guest access, changing the phone's wireless advanced settings sleep policy to never and setting the battery mode to Performance mode.
Often battery saving apps switch off WiFi when not connected to save battery.
Many applications that perform network operations in the background use a receiver to run when the network connection comes up.
Thus, connecting to a network each time, it turn the Wi-Fi on, all these apps will run, drawing more power.
On each toggle on and connect, mobile devices consumes more power.
If the Wi-Fi was on all the time, applications might run more or fewer times to update or sync data. This very much depends on the apps installed/used in the device.
If user is turning off the device turning the screen on just to turn on Wi-Fi and check for messages, that'll use more power than the Wi-Fi itself.
Turning the wifi on and off is more energy efficient than leaving it on all the time. To statistically prove this since the power consumption is influenced by other factors. For better battery life experts recommend greenify application, turn off all location services and disable the always allow network scan in the wifi settings.
It is possible that battery from an Android device is pulled out without switching off the Operating system.
Often, the touchscreen of phone stops working completely. So, in that case, it is required to pull out the battery because without the touchscreen phone can't switch off the OS.
It could result in user working on files that were not saved, they will be lost.
No proper shutdown means that the OS and other apps in use and were using some files, plus the file system was in use. They can become corrupted and in worse scase will be the OS itself to get corrupted. For example, if user were updating the OS, and for whatever reason, you had to remove the battery. This could lead to losing data files, even break the device.
Some phones may lose the date and time settings. Application updates do not recognise System OTA Updates.
There are several problems due to the fact that still use older Android and updating an application built for Android 8.0 or other will not work on device.
The application will install the update, however the Data is still configured for older version, so the application can not read its own data.
Uninstalling the application might erase the data and allow a new installation to initialize the data for a newer configuration.
If the application stores most of its data online or in the cloud, through an account, then uninstall/reinstall isn't actually fine. Upon reinstallation is sign in and the app should pull all your content back from the cloud.
If user do a normal system update and no restore etc, then your apps are not uninstalled. The system doesn't modify apps at all until it finished the system update. Then the system will recompile all apps in order to make them run faster. This should not interfere in any way with application data.
The Android permissions model has been evolving with each release of OS. Starting in Android P, apps can request directory access to receive permission to access a scoped directory on the device.
For example, an app that requests the permission WRITE_EXTERNAL_STORAGE gets full access to external storage.
However, using scoped access, an app that takes and manages pictures might request access to the Pictures folder on your external storage device. This allows you to ensure that the app in question can read/write data to Pictures or other directory.
Android releases frequent Security updates to address a number of vulnerabilities at operating system and hardware level. So as a thumb rule, the newer the better and safer. But if you just want to use hotspot, it's not very difficult to enforce the basic security measures.
Using as hotspot means your phone is working as a router. Routers do have builtin firewalls for better protection, and some advanced features like Port Forwarding, DMZ, UPnP, QoS, VPN, multiple AP's, RADIUS, NTP, DHCP server, DNS server, Dynamic DNS, VoIP features, captive portal, traffic sniffing / DPI, anti-malware etc.
A normal Android phone offers some of these features out of the box; the basic NAT, netfilter (iptables) firewall, Access Point (AP) with WPA2 encryption and a DHCP/DNS server.
Additionally if user is tech-savvy, you can also define your own firewall rules, port forwarding rules, setup a VPN etc.
User often complaints that device is a few years old and the last official update was few years ago. This means device can not get security patches.
Manufacturer does not provide updates, then user have to get a custom ROM, available online, or get a new device.
Trust that the custom ROM does not brick your device or destroy smarthone components.
The more people use a specific custom ROM the more likely all serious problems are known and may be even already fixed. For that reason, user prefer LineageOS based roms without any modifications or the minimum to support the specific device.
Custom ROMS contain a lot of compiled code. Especially the kernel and other sensitive software running with high privileges is usually compiled by the author. Even if the sources for the kernel used by the ROM are public and it is a common problem to prove that the source code is really the one used to build the kernel.
Detecting something malicious in a kernel or a different compiled software is very problematic.
One common problem is for example that custom ROMS that are signed with AOSP private key. Therefore it is easy for app developers to sign an app with the system keys to gain special system permissions.
The permissions structures are dramatically different between different android versions. Upgrading allows it to recognize the older structure and alter it appropriately for the new requirement. User can copy all the data off of that card, format it in the phone, and then copy it to device.
Many system apps have their original apk stored on the system partition, which as it is fixed in size wouldn't really count towards available storage. Nor are they user removable. So it doesn't make a lot of sense to display a size for them, when they are not consuming any fungible resources which a user could chose to put to a different use instead.
And if pre-dexed as is typical for things packaged in a system build, they don't need those lookup resolutions against the installed system libraries stored in a runtime-writeable place either, something that is ordinarily a cost of having a latent downloaded or sideloaded app installed.
When the system apps are used, they create some data and cache, for which their size becomes a few kilobytes only
Indeed, files created during use cannot be stored on the system partition, but only on the data or cache partitions, competing with what other apps need to store there.
All of that is not to say that some other configuration might not report this differently, or that some other system apps may consume much resources by the time the user can interact with the system.