I don't understand Ranked.

Sometimes I really don't understand how Ranked decides who to place with who. I'm Silver 5 and by far not Challenger. But why on Earth am I being placed with Bronze 5s with 500 mmr? That isn't remotely close to my elo, which my mmr fluctuates between Bronze 1 and Silver 4. I thought we were supposed to be placed with people who are in our elo, but I consistently get placed with people who are bronze 3 4 and 5 on a regular basis who just end up feeding or being stupid. I'm not the perfect player, and I make mistakes too.

We all have bad games, but THESE ARE CONSISTENT. Why am I being put with people that far below my elo? I just don't get it. Do I have it all wrong? Are you just placed willy nilly? I'm quite confused. These past 2 weeks alone I went from promos to like 12 lp because of Bronze 5 feeders in every game. It just goes down hill from there. I'll be 6/0 and suddenly I'm 6/9 and we are down 2 inhibs and top lane is crying about me when he is 1/7 and Bronze 4 with a kda of 1.0 and a 20% winrate. Maybe I'm just pissed, but I genuinely don't understand how this thing works.

I've tried different roles, thought maybe if i can supp my team we can win. Nope. I'll get botlane 7/0 and team fight comes and all hell breaks loose because everyone is more passive than my grandma's sex life. And she's dead. I just don't know anymore. I've been shielded from demotion more times than I can count and I'm kinda getting tired of it.