Robb Hi, and thanks for your interest in using Ziva VFX!
I'll let @andy_vs chime in here, in case I'm overlooking something, but I don't think that we have a specific tutorial that specifically covers the muscle modelling process as of yet.
We did record a webinar though that will likely have some useful information in it for you, here: https://community.zivadynamics.com/d/455-modeling-muscles-over-skeleton
There is also another old thread in the community that might be worth reading through here:
The above thread was started quite a long time ago now though, so our process has evolved with our toolset (we no longer require a seperate fascia simulation for example), but most of the actually modelling tasks as still the same.
Regarding tools that we use to model our bones and muscles . . .
Here are some of our favorite: Maya, 3D Coat, ZBrush and Meshmixer.
I hope this is of some help!