Non app storage needs then... My best guess then is messaging app attachments? maybe OP needs to save docs/pdfs he gets from Whatsapp/telegram. But you're right, that seems more of a laptop/PC activity.
Immich for pictures seems like a good answer to your question. What's files: Mostly pictures I think. Why not just keep it on phone? people love their pictures. I have more than a phones worth.
My goal was to use something other than TikTok, facebook, etc.. Even with all its glitches and quirks, the FV has proven to be a great replacement for those corporate apps.
It's their birth right