How does hidden MMR work in League?
Riot's matchmaking rating (MMR) is invisible to players, but it governs every queue: it decides who you play with and against, and it feeds into where you place when ranked unlocks.
Each match moves your MMR up after a win and down after a loss.
A new account starts from a default rating; an account that keeps winning climbs above it and meets stronger opponents. Riot does not publish MMR values or exactly how normal games carry into placements, so a high win rate is a head start, not a guaranteed rank.