How exactly do promotions work?

Story time first : So after a bunch of normals, winning/losing, my MMR got to the point of me being matched with plats and golds, so i decided, heck lets do ranked. Promotions were super rough, everyone was bronze and had no idea what to do most of the time, people were quitting and raging and flaming, it was chaos. Still, after much much hard dedication i got through, with 6 wins out of 10. Alright so as far as i know, 6 wins is either bronze 1 or silver 5, but what i got was bronze 4 (yay i can brag im not 5), last season i was bronze too, so how exactly are you put into a league after promotions? Does it just count yours for that season or count your previous rank aswell? Thank you!