Ok, so I've taken a closer look at the covers and this is what I've learned.
Each cover has 9 pieces. Assembling a whole cover will give you a stat boost and some kind of reward, like bios.
You get cover pieces from buying cover piece chests for 50 gems, or as semi-random drops in Dimension Rifts.
You can trade two cover pieces in for one random new cover piece.
Here's the shittiest part: The Captain Marvel Carol Corps costume is 1) time limited, and only available for the next 31 days, and 2) locked behind four PREMIUM covers. The only way to get premium cover pieces is through the RNG. Either by fusing cover pieces or from the cover piece chest.
4 premium covers x 9 pieces = 36 different cover pieces that you have to pray will drop some time in the next month if you want that costume. If you have the most amazing luck in the world, you will need at least 72 cover pieces to do this, or spend gems. Each dimension rift run so far has given me one cover piece.
I don't like this at all.