The interface (API) is dictated mostly by how DMM (Dream Multimedia) created it in their implementations of enigma2 and the drivers. The other vendors will have to follow this design for existing functionality. For new features they can choose the interface (which will mostly be dictated by Broadcom anyway), which, in theory, other vendors will need to follow. We at OpenPLi are a bit more strict in this than other images builders, which tend to allow for all sorts of exception code for different vendors. We try to avoid that as much a possible.
This concept is the same for Broadcom, HiSillicon and any other SoC manufacturer. We don't care what SoC is inside, as long as it plays by the rules and then it will be no problem to build an image for it.
The API consists mainly of the well-known DVB4Linux API, which has been in the kernel for ages. Quite a bit of functionality is not covered there, though and has alternative access, like additional ioctls and /proc (/proc/stb/...) entries. AFAIK there are no driver implementations yet that use /sys.
If you want to know how it works and how it was designed, I can only suggest: "Use the source Luke"! The original implementation of Enigma2 by DMM wasn't really documented and everyone of us (image builders) have kept to that tradition. Let me warn you: it's not pretty.