MMR

From what i know its wins and who your winning/losing against. so say your personal MMR is S5 and your vs S3 if you keep winning vs S3s you will end up rasing your mmr and end up vs S2s and S1s and the LP you gain from winning will go up because the system thinks its too easy for you so riot wants you out of that elo as soon as possible so they give you more LP. So if your S5 and you end up vs S1s and winning you will end up getting like +29 or something and if you are S5 and end up vs a full tier above you or so maybe around S1 and G5 you will end up skipping from S5 to S3 because again the system wants you out of that elo. Same thing happens with loses If you S5 and end up losing alot to B2-B3 you will demote because it tanks your MMR

Maybe someone else knows more then me but this is what i see from my first season actually trying

ok thanks alot

MMR goes up when you win games, and down when you lose games.

Your actual performance in the game doesn't matter at all, only if you win or lose.

How much your MMR goes up/down depends on your mmr relative to those in the game with you. If you're playing solo, things are usually relatively even MMR wise, so you'd gain about as much MMR for winning as you'd lose for losing. However, if for example you're in a premade with someone with a much higher MMR than you, you'd be matchmade based on your average mmr, meaning you'd be in a game where most players are somewhat higher MMR than you. You'd gian a lot of MMR for a win there, but you wouldn't lose much MMR if you lost, since the system 'expects' you to lose in that scenario. The reverse is true if you go into a premade with someone much lower MMR than you, where you'd gain a tiny amount of mmr for winning but lose a lot if you lost, since the system expects you to be able to carry.