As a student of life I have to mention Anki. At its core it’s a flashcard app out of the box but it’s almost infinitely more powerful than that if you dig deep.
I paid 35 CAD for it in 2019 and I’ve used it every single day since. The amount of knowledge I’ve committed to memory is truly priceless, and I’ve even gifted this app to several friends.
NB: AnkiMobile, not the free knockoff AnkiApp. It’s open-source and actually free for computers and android devices, and iOS/iPadOS app purchases are the only way the developer makes money.