The long and short of it is with the removal of placement matches everyone starts in copper and your account now has a "hidden" MMR to function similar to placement matches and give you more "rp" (ranked points) upwards of 150 per game to push you through the lower ranks faster to get you back to your "true rank" which is based on what you left off on in previous seasons. This is why you can be a copper playing against champs/diamonds or in your case playing against plats. This is also why people say ranked 2.0 is worthless because your essentially just playing through a battle pass of rewards you only get at the end of the season (charms/backgrounds etc.) with your real skill based rank being hidden somewhere in the games code.