Spaced repetition and Anki
Spaced repetition is a technique for memorization that shows you flashcards at intervals based on how well you know them. Rather than surfacing a flashcard at a fixed frequency, it will present it at exponentially increasing intervals, because apparently the strength of a memory operates on a similar principle. This means you have to see each card fewer times and review fewer cards each day for the same memorization outcomes.
Anki is the most popular app for spaced repetition. I used it for a couple years to study Japanese. It works really, really well; I was able to memorize ~10k Japanese words in less than a year, and I still remember the majority of them 7 ish years later (although of course I saw them outside of Anki and since I stopped using Anki regularly).
I recently saw Mochi mentioned on Twitter as an alternative to Anki. I haven’t tried it out yet but I want to soon!