Call of Duty: Black Ops 6 uses hidden data to determine which game lobby players belong in. However, there is a way players can find out their rating. Black Ops 6, Warzone, and previous installments of the Call of Duty franchise use what’s called skill-based matchmaking (SBMM). Each player is given an SBMM rating based on multiple aspects of their in-game performance, which then determines who they play against. In theory, this keeps multiplayer games more balanced than if matchmaking was done completely at random.

https://screenrant.com/how-to-get-sbmm-call-of-duty/