How preseason, MMR and placement related ?

With these new ranking placement and position ranks, I am kinda confused.

1. Does my rank/MMR from preseason affect my first placement ? What else affects first placement ? I just can't get how can you place a player right from the first game. If he gets his ass carried he can get Gold 1 and if he gets trolls playing well he can get Iron 3 ? I really want to know how it works.
2. Now does my preseason rank/MMR affect each position or only the first one played ?
3. Does my position rank/MMR placement games affect my other positions placement ?