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.
https://www.youtube.com/watch?v=pLPpl2ISKTg