I asked part of this question in the recording and mixing forum but this is specifically the bit about subgroups in the RB mixer.

if i send all the backing tracks to a subgroup can i put one plugin fx on the subgroup channel strip instead of individual channels saving plugin resource needed. would it act like a mini master bus? i'd like to try whether i can globally process part of the mix by using a subgroup.

and if i have reverb on an aux send and return, and route several tracks to it, will the reverb be affected if i send tracks to the subgroup? presumably the aux return sends to the master output bus (as i think a hardware mixer would do) so it would bypass the subgroup which could mean a reverb level at the original track volume setting not a reduced volume if the subgroup fade is lowered. or would it?

maybe i'm just getting too clever for my own good with nothing to do in lockdown...........

They are separate paths and then summed when at the master out (in my experience)

Tracks that get sent to the AUX (with FX) get sent to the Main OUT. But so does the original track output (with or without FX)
Subgroups also get sent to the Main Out and get joined with AUX and other track sends (those not in the subgroup) but the tracks go to the subgroup first before being sent along to the main out.

The difference is an AUX send 'Adds' the effect to the original track (which the AUX receives dry), which then gets sent to the main out in addition to the original track's output.
When using Subgroups, the tracks go to the subgroup first (with any FX added) and then sent to the main out (so they intercept the track, affect it, and send it on its way). In other words one is a parallel path, the other is a linear path for the original Track output.

At least that is how I understand it works.

With an Aux path both the Track and AUX go there as separate entities.
With subgroups, the track(s) in the subgroup go to the subgroup first before being sent to the main out.

well - a bit of trial an error. yes you can put a plugin on a subgroup and use it like a mini master bus - i used neutron elements with a main mix preset. the track assistant feature listened to the subgroup and made its suggestion.

bur rharv is right - the PG aux sends its return to the main output bus. on a hardware mixer you can reroute the aux return to the subgroup so the effected signal is changed with the subgroup slider in line with the original signal.

that doesn't happen with the RB mixer. i sent two tracks to a subgroup. the subgroup fader does not affect the track faders - they don't move. i then soloed one of the tracks and took the subgroup fader down to mute the soloed track i still got an audio output of the reverb fx sent on the aux return to the main output bus.

so looks like you can't use aux send and returns with subgroups - the fx return does not respond to subgroup volume slider changes.

i'll put a wish list post on the RB wish list forum. two useful facilities don't work together.

and no I've not get anything better to do during lockdown..........