Also Localsend for quickly transferring files to/from android and other devices over the network is really useful. Its a flutter app so the interface is very consistent and clean on all versions, and works a lot better than warpinator or other similar tools that ive tried.
Not really an android app, but better-adb-sync is also very useful on occasion for transferring large files quickly to a computer over a cable. As the name implies it uses adb for file transfer rather than mtp or any network protocol. It is very fast!