How does matchmaking work?

So every time I get to my gold 1 promo I get the following:

• trolls ( i go mid then support follows me mid and accuses me of trolling)
• multiple people with high ping that they basically afk and it becomes 4v5 or 3v5
• people who lose upwards to 5-10 games in a row and obviously tilted

I just want to know how does matchmaking works cuz I don't get how I am only paired up with the above ONLY in promos. 6th promo to plat lost and all 6 failures were 0-3 not even close games