Most PT schools have a direct PT shadowing/volunteering requirement (i.e. 50 hours of PT observation experience). Some may require multiple settings (inpatient, outpatient, peds, neuro, etc.), whereas some will be fine with 50 hours in an outpatient clinic.
It's not a bad idea to shadow other health care professionals to get an idea of what they do (i.e. I observed a surgery and I also volunteered at an animal rehab clinic, and I know people who shadowed PAs or orthos), but it's usually not required by PT schools, and won't count toward your PT volunteer hours requirement. But, there is a place to list non-PT volunteer experiences on the app, so the schools will see you did it, it just won't count for the PT requirement.
And I think you're confusing volunteering versus shadowing...they're basically the same thing as far as I know...not all clinics will let you "volunteer" in the sense that you are actually doing something there other than observing. But observation experience is totally fine on the application, as is paid work experience if you work in a PT clinic. It doesn't have to be strictly a "volunteer" position.
Does that help?