It's not random, in regards to what champ you get, the pool includes:
-All available champs owned by the team
-Free week champs
and individually, you can trade between the team, and reroll 2x for better champs. The results are absurd-- in any game, like, half the champs will be A or S tier for the map, and in my experience, concentrated heavily on one team.
God forbid you do well on a champ you don't own though, any rewards earned through the loot system will not be eligible because you should've bought RP and bought the champ, you rancid free-loading piece of shit.
Disregard that we called it the "mastery" system and not the "own-the-champ" system, but I digress.