Firebase authentication+ Backend in mysql

Hello,
I am working on a new app. I want to keep firebase for authentication because it is simple but I want a real separated backend (db=mysql) with API connections.


My question is what is the best solution to link the information I have in mysql and the authenticated user from firebase ?


Example:I have a table with pictures in it.I want to retrieve all pictures which belong to me (as the connected user). So I need a link authUser->table pic.

Before doing anything rash, I prefer to ask.

thank you in advance