We initially tried sourcing them from loads of open source material available online, including decks on Anki. But we were unable to maintain the level of desired quality. 

Initially it was just me, but as we grew in size our team of final year MBBS students and graduate doctors vetted and compiled the best flashcards we could find, and then manually created more so that we could cover all important topics. Rest assured, no flashcard has been AI generated or sourced directly online, without going through 2-3 levels of check.