Learn Japanese: Bunpo
Learn Japanese: Bunpo provides simple and clear grammar explanations from JLPT N5 to N1. The app is not intended to be used alone in order to become fluent in Japanese — rather, it is a support for those who want to have a better grasp of Japanese grammar. A good portion of each section is spent on quizzes to test your understanding (although some people may still find that they advance a bit too quickly).
There are some improvements that could be made, but overall Learn Japanese: Bunpo seems to be an effective resource for anyone who wants their grammar practice in one place.
Learners new to Japanese can enjoy a free introductory level that will provide you with a basic foundation of vocabulary, hiragana, and katakana. For N1-N5 learners, you can try the first section from each of the levels without making any purchases. The paid version includes SRS review, listening exercises, and chat functions with native speakers for support. You can choose to pay monthly, but the Lifetime option is more economical.