Can someone explain me how promos matchmaking work?

I can't undestant how promos work no joking, i was on promos to silver III and got a adc level 31 who lost all of his ranked game since he started rank no joking he lost near of 9 games of first 10 games placement. Logic we lost bot cause he was a inexperienced adc. This hapens on all of my promos some games win other lost but always people on placements go finish on my promos, while i get Lp to reach 100 i get none guy on placements when i reach promos is all games. So if someone know how matchmaking work on promos i dotn understant why the constant people on placements on promos and i'm not the first complaining about. Does riot do intentional to you need carry they, it looks like cause is all time on promos can't be lucky, i only get that promos game that hapend last 7 promos. Why not place people on placements vs people on placements? or Why not place all account on iron 4 and if they are good they will rank up? Silver aren't good player neiter i'm but place a level 30 new player on silver games is hard to deal. Sry for post i'm probably just venting cause i got mad cause that,
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