the masterbus/mixbus/2bus chain is just opinion and mixing style dependend. some mixers start with the mixbus and end on the individual channels (top down approach to mixing); me on the other hand start from the single tracks and i go up in the bus hirarchy.
for me the mixbus is my genre and song dependend last phase of mixing. i compare my mix that i made until then (you can read about my whole mixing process here if you like to: https://www.producelikeapro.com/forums/topic/where-do-you-start/page/2/) with my references and decide which steps are needed to make it a "radioready" mix. for example if i wanna have more low end and i would just turn up the kick and bass, then the click of the kick would also get louder, as would my presence of the bass. if i don't wanna have these things louder but only the low end (and for sure you have to high pass correctly for this!) then i can put an eq on the mixbus and crank up that low end.
if i hear that somethings just not working mixwise but can't say which track makes this problem as a last resort you can always adress these things on the mixbus (again as a last resort!)
so my usual mixbus chain is: Virtual Mixbuss Emulation, EQ, Exciter, Buss Compressor, EQ, Tape Emulation, Limiter. how i process things through these is depending on every song.
so for me the mixbus stage is the point in the mix to get a good mix to a great exciting mix.