I believe how well you play is taken into consideration, even if you lose the match. I lost 5 of the 10 placement matches, yet still placed Platinum 1.
There are (I think) 6 tiers in each rank. If you’re Platinum 1, you will need to reach Platinum 6 before you can reach Diamond 1.
you only rank up if you win, but if you preform well you can hold your position without dropping. so just play well and win when you can and your climb. Dont play well combined with losing more than you win and your go down until your matched with people your skill level.