It was a huge cluster (30 large servers in total) of MySQL with the MyISAM engine running in a Master-Master configuration. No foreign-keys allowed, the Apps were responsible for correctly enforcing data constraints.

Nowadays it seems the answer has somehow become "Pay MongoDB a ton of money for a support contract" and call it a day (Fortnite by Epic Games). Let's just say this isn't really my style, but somehow the game does work. To be real with you, keeping track of player scores, doing lobby matchmaking, and storing a few hundred or thousand items is pretty straightforward, even at "high-scale".