RIOT never explain how this worked, I went 6/4, Bronze one, my friend went 5/5 SIlver III, Last season we both Silvers.
They explain it reasonably well, they are just light on the numbers and the information can be hard to find.
Each player has an MMR (MatchMaking Rating -- very similar to an Elo rating) that determines how skilled the matchmaking system thinks you are in that queue. As a gross simplification, your MMR determines what "rank" you are.
Each Ranked season (so far), Riot has done a "soft reset" where they move everyone's rating closer to the median rating (which is somewhere around low Silver) when the new season begins. If you have never played Ranked before, they start you at the median rating.
Each match you win increases your MMR, and each one you lose decreases it. Placement matches simply have a larger adjustment per match than regular ones, so that (in theory, anyway) it gets you closer to your correct rating more quickly.
My KDA was PRETTY great, his were just meh.
In-game stats are never taken into account for MMR in any game mode. Ever. All that matters is whether your team wins or loses, and the teams' relative MMRs. You gain more and lose less when you are the underdog, and gain less/lose more when you are the favorite. Otherwise Riot would reward certain playstyles and strategies over others, even if they results in the same number of wins and losses.
He waited a while before he played placement (like 2 months after) while I played as soon as it restarted. Does that have anything to do?
No, it shouldn't matter.
What divisions did you end in last season? Did you play in the offseason? Offseason matches continue to affect your MMR, so if you goofed around and lost a lot in the offseason then that will cause you to place lower when the new season starts.
