You can download the APK once from the website, after that it updates automatically. https://signal.org/android/apk/

I like to use Anki with some shared card decks: https://ankiweb.net/shared/decks?search=german

The learning pattern that Anki has configured by default suites me quite well and the app and the card deck is highly customizable.

Mind you, this is mostly only for learning vocabulary, not for learning grammer.

I don’t know where you got “a couple of years old” from. Do you mean, that the Fairphone 4 in general is a couple of years old?

Because, on the website it says the refurbished devices are: “After less than a month of Fairphoning, the phone is returned” So the Fairphone was only in use for max 4 weeks. I would still count that as mostly first hand :)