I was just trying to figure this out myself and put a suggestion on https://itch.io/t/1818926/where-do-i-put-carts-on-android#post-7132064
The app should have an Android intent that lets other apps, such as a file manager or the web browser after download, share a .tic file to it so you can open files that way. Not sure if the app also needs file system permission with that or if the intent system just passes the object without necessitating that (probably?) Or the app could have file system permission and create the TIC-80 folder at /storage/emulated/0/TIC-80 instead. That way users can copy files from another machine and play them.