3/
"F-Droid" also comes pre-installed. F-Droid lets you install open-source Android apps.
Yes, the FLX1s can also run (many) Android apps.
You would install Android apps if you cannot find a native version or alternative in "Software".
This is where you would look second, if you cannot find what you want in "Software".
4/
You install "Aurora Store" from "F-Droid". It lets you install non-open-source Android apps. Ones that you cannot live without and cannot "get" any other way.
Again, yes, the FLX1s can also run (many) Android apps.
This is where you would look last, if you cannot find what you want in "Software" or "F-Droid".