Basically I am building a social media type app. I want to manage the auth using firebase firestore (as It is easier) and the contents posted by the user in any other SQL database. Like, if the user uploads a image it will be stored in my own database. Is it possible to link firebase with my own database? If yes, then how?
Firebase Authentication isn't related in any way to Cloud Firestore. You can authenticate your users with Firebase and save the data in any database you want, even in an SQL database. However, Cloud Firestore has some benefits. So I recommend you check the official docs.