How does Riot choose who I am going to play in ranked/normal?

So basically I made another account, a smurf, and that account keeps playing against mostly gold players and some plats as well. I basically feed half of those games, but the other half I do ok in. Why am I not playing lower opponents, playing a Plat 3 yasuo as an unranked ahri is kinda bs. Also my first placement game was with 7 silvers (one S3, 4 S2, and 2 S1) and 2 Gold V's. Am I actually good enough to play with these guys, or is the game just inflating my MMR? You can op.gg my match history to see.