Inside the apple ecosystem

And there's the rub. The entire world doesn't use Apple. I need to be able to plug it in at work or something, have the Windows PC there automatically recognize and mount it, drag files to it, take it home, and then plug it into my Linux box and grab the files.

As for your second question, it's usually "naughty" apps like Youtube Vanced, IT utilities and tools made by individuals who only offer an APK download and aren't in the store, or my own little hobby project apps.