Where are photo files stored on mac in Lightroom CC?

As johan says. But newly added photos are temporarily stored in the cache. You can find the library in Pictures on Mac OS. LR CC gives you the option to use the space saving dynamic cache (default) or to store a cloud linked local copy.

jjjnyt  wrote Pretty sure in Classic version this was as simple as right-clicking the image and choosing "show in Finder" but can't seem to figure out how to do this in Lightroom CC. If you tell me they are in the cloud you're going to have to be more precise than that.

They are in the cloud. How could I be more precise than that? This is the fundamental difference between Lightroom Classic CC and Lightroom CC. Lightroom Classic stores your photos locally, Lightroom CC stores them in the cloud on the Adobe servers.