Which files does `adb backup -apk -shared -all -f  back up?
Answers:

No,  adb backup will only backup the applications that has Backup option enabled.
It depends on which data
Yes, you use can connect to ADB over TCP (https://stackoverflow.com/questions/2604727/how-can-i-connect-to-android-with-adb-over-tcp).

Details

List applications that probably can be backed up:

adb shell 
for package in $(pm list packages -3 | cut -f2 -d&quot;:&quot;); do
dumpsys package $package | grep &quot;ALLOW_BACKUP&quot; &gt; /dev/null  &amp;&amp; echo $package;
done

Probably: WhatsApp has ALLOW_BACKUP flag but cannot be backed up with ADB.

Verify the backup:

To verify the application has been backed up successfully, you can use the Android backup extractor tool.

Storage backup:

adb pull /storage


Backup SMS:

adb pull content query --uri content://sms


Backup contacts:

adb shell content query --uri content://com.android.contacts/data