I’m seeing a lot of wrong things said in this thread. Here’s how it actually works.
Highest visible rank player always bans, unless all 5 players are still in placements in which case it’s highest hidden MMR.
If you dodge a game it only lowers your visible Rank Points and does not affect your MMR. The -500 penalty stacks infinitely. If you do placements for a new season your whole leaver pool from the past season is earned back, any you didn’t already earn back, because you place according to MMR. I know this because I tested it recently. I had a gold MMR account that I dodged all the way down to Bronze 4 on (playing a lot of leaver games). It was still getting gold games and after new season placements it placed gold. I did the same thing again next season, winning a lot but dodging a lot so it stayed gold while getting diamond games and then placed diamond the following season (this one).
By the way placement games are just normal ranked games. You just move the usual ±200 (500 or more if you took a long time off the game, around a month, or if you’re on a long win or loss streak) from your MMR at the end of the previous season.
Rank decay (that thing that happens in diamond+ if inactive) only affects placed accounts at that rank and only if your visible rank is that high. Also it is both rank and MMR decay except there’s a bug where it doesn’t become real until you play a game post decay (and that game will happen at the new decayed rank too).
So you can decay from high Master to D5 and it’s wiped away next season with placements so long as you don’t play any games after decay because if you play even 1 then it’s tied to the rank at the time of that game decay included.
However back to dodging, the matchmaker considers both rank and MMR at least on major regions (the NA, EU, and probably KR servers). On all other servers it only tries to average MMR but on major servers it both tries to average MMR for each team AND restrict players from matching together if they are too far apart in rank. If all players were solo they would generally find games only with people within 2 rank divisions of each other. After a very long time in queue (possibly an hour or more) then within 4 rank divisions. For Master it’s slightly more tolerant I think.
When parties are involved it’s more complicated, I think the 2 or 4 division thing applies beyond either end of the lowest and highest rank person of the party on your team.
Point is, if you dodge too many games it eventually becomes almost impossible to find games on the major servers (can only match with other pathological dodgers or high rank spread parties) unless you yourself play in a party.