This has been bugging me since I took AP Euro in high school. The Medici family made their money as bankers, but I was taught that Catholics couldn’t charge interest on loans.
/u/DeSoulis gave an answer to a similar question over the how they got around the ban on usury.
This older answer by /u/alvisefailer answers when the Roman Catholic church allowed banking
https://www.reddit.com/r/AskHistorians/comments/3gp24c/the_medici_bank_vs_usury/
However they don't answer how the Medici ended up doing business handling the Catholic church money so more can always be written.