I’m not from a commercial aviation background but I assumed that, in general, crews Flew as “a crew” and new each other?
No, it is not possible. Remember, airlines are pretty huge and the logistic problems to always crew the same together is pretty huge. I work in a pretty small airline, but we still have around 1200 pilots in 10 different bases and (currently still) three different fleets, although there is some overlap between the MFF and the pure narrowbody airbus fleet. On my old fleet i knew all the guys on my base (around 60 crews), but even then it was pretty common to fly with someone from out of base, it was the norm during simulator events. Now i'm on a new fleet and nearly every time i turn up for work i get to know someone new. That is actually a lot of fun to and probably will continue for the next two years or so, especially as crews from closed bases will be relocated to mine.
I heard from a colleague that the old GDR Interflug operated a team crewing system where small teams were crewed together, dunno how that worked out, but they had many more persons on the flightdeck to begin with.
There is a possibility via preferential bidding to bid for a certain partner, either flight deck or cabin crew and in theory those should be crewed together more than normal.