Thank you! I used Realtime Database from Firebase. It was my first time doing something like this, and my friend suggested me to try this platform. Overall, it wasn't really difficult, but there were some issues with the web version of the game... Anyway, if you're interested, you can easily find tutorials on YouTube.
(Specifically, in my game, I just create unique IDs for items and store them in the database along with other user information. I don't upload any files to the database, only text data).