How to access variables from other games

My problem is, I want to be able to access a variable in one unity game, from another unity game, both that I own and made.

I’m trying to make a mobile app in unity, to go along with my unity game. The game will tell me players, allow me to use and access variables and/or voids/functions from within the mobile app.

Is there some way of doing this?

My idea is like a stat tracker abit like Fortnite Tracker (The Website). Users can login and see all of their stats.

I’d say create a Firebase database