To me it looks like there are 3 sets of coders. Non FSD updates, FSD updates with 11.3.6 and FSD updates with 11.x.x. To me it seems like they are
trying to herd cats. As far as I;m concerned there should be 1 set of code that has logic to determine HWx, FSD, non FSD.
I believe that it's just two. The infotainment team and the FDSb team.
This is pure speculation, but I'm guessing that the infotainment team had their software working with the older autonomy software and that the two pieces of software interacted via a certain interface. FSDb uses a different interface, so each time the FSDb team wanted to release new infotainment software, they had to retrofit the infotainment stuff to their interface. They didn't want to do that often, so infotainment updates were rare.
Now that FSDb has stabilized enough, Tesla decided that the infotainment team could modify the official infotainment software to use the FSDb interface. That gave them access to FSDb 11.3.6, which they included in their next update as a kind of proof of concept.
Given that scenario, so long as the FSDb team doesn't fiddle with the interface too much, the two teams can at least hand off software to each other more easily. The two teams may not ever work on the same release because of timing issues, but at least they would be able to trivially use the other's work. It would mean that there would be a single version line, but with different releases having either an infotainment update or an autonomy update, depending on which team just completed a set of changes.
A different theory about the interface thing is that the original software didn't have a defined interface at all, and that the infotainment stuff was all intertwined with the autonomy stuff. Given that theory, the latest work to include FSDb 11.3.6 in the infotainment line was all about getting the infotainment stuff to have a clean separation from the autonomy software. This may be the more likely scenario given how fast Tesla works, and how much of an unknown all this is.