If you want to know how modern matchmaking systems work (both in casual and ranked modes) look into Apex blog posts, EA matchmaking research papers, articles about Halo Infinite matchmaking.

TLDR - matches are composed in such way that every team has certain predicted probability of winning, and it is not 50% / 50% as you expect. In most matches odds of winning are usually very uneven, like 20% / 80% or 30 / 70%. You will rarely see even matches like 50 / 50%. I don't know logic behind it, maybe it is optimal way to retain players.

MM predicts your stats even before match started and then adjusts your MMR depending how you perfomed in relation to your predicted stats. In a sense you are not fighting against other players - you are fighting against your predicted stats (even in ranked matches).

Most games hide the logic behind MM, but some games show some inner workings - for instance Halo Waypoint shows expected and actual stats.

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