There used to be a great site maintained by Robert Monaghan about all things Bronica, but it's kind of become dispersed. If you google, you'll find bits and pieces of it, but maybe the whole thing is preserved in one of the internet archives like the "wayback machine."
Basically, nothing from the classic Bronica systems (D,Z,S,C,S2,S2a,EC,EC-TL) is compatable with the later systems (ETR,SQ,GS). In the older systems, generally lenses are interchangeable, and some but not all backs and finders are compatable between models. The earlier models had a built in focusing mechanism, but once the separate helical was added that was compatible with all the later classic models. Bellows vary, and there are adapters between systems, as well as tricks for using an S2a bellows on an EC-TL.