Any tutorial for storing user-specific data online (e.g. unlocked cards)?

I’m trying to make an RTS card game (like Chess x Clash Royale or TFT), and I need a way to track what cards a player owns. Owned cards are a sensitive part of game balance, so I can’t store the information on the player’s machine. Ideally, it would be linked to their Steam account, but they wouldn’t be able to access or change it. What would be the best way to do this? And are there any tutorials for it? I really have no clue where to start.

Thanks in advance!