CF = content framework, android somehow decided that users shall not see and interact with "real" files and instead, have apps like nextcloud act like content providers and expose a file-like API ...whatever, it is what it is, but in the end it works.

I'm currently using Material Files, but even android's default file manager, bundled with the OS, shows Nextcloud in the left sidebar (your mileage may vary on this one, as each phone vendor tend to customize it a bit).

As for my setup, there's really not much to it: I selfhost nextcloud, have KeePassDX and the Nextcloud app, and when you setup KeePassDX, select "Open existing vault" and in the sidebar you should be able to select Nextcloud and pick files from there.