Probably the spaced repetition application SuperMemo [1]. Beyond normal SRS like anki it has a feature called incremental reading [2] which makes studying at least 3 times as easy for me. I have ADHD so IR helps a lot by taking care of 90% of the complexity of (managing) studying so that I can just focus on what's in front of me.

[1] https://www.supermemo.com/archives1990-2015/english/smintro [2] https://supermemo.guru/wiki/Incremental_reading