Your MMR (Matchmaking Rank) is just a single number. It’s basically equal to your SR, assuming you aren’t Diamond+ ranked and suffering from SR decay.
What map it is, what map type, whether you start attacking or defending, all those things are irrelevant. Which does, yes, mean that you’re expected to perform at the same level on Payload as on Control, even though you might actually be better at one or the other. You also might play heroes who are better suited to certain maps and perform better or worse depending on that, but that’s all the same to the Matchmaker.
Which map you play is mostly random, maybe completely random. It has been suggested that the matchmaker tries not to put you on the same map twice in a row, which would be reasonably easy to do (take a list of all maps, remove the last played map of each player from that list, pick randomly from the rest). It has also been suggested that the matchmaker tries to make sure you see all maps and gives a map priority if you haven’t seen it in a long time. I guess that’s not impossible, you could look for the map not seen for the longest time among all 12 players, and pick that one or pick randomly among any ties.
Anyway, matchmaking does not take into account really anything other than MMR, group sizes, and how long you’ve been in the queue. It tries to find 12 people with similar MMR and similar group sizes, but it gets less strict the longer you’re in the queue.
There’s all kinds of details collected here (on SR/MMR, not on map selection): How SR works (S16)