As others have mentioned, the big three (ATT, VZW, T-Mob) all have independent hardware (antennas, transceivers) that just co-locate on the same towers. MVNOs lease capacity from them and have contracts that vary wildly but for the most part don't have the full functionality of the big 3. In some locations there are more than just the big three (like US Cellular which I think still maintains their own network).

I have a friend who manages a company that services the HVAC units for cell towers. He took me on a tour once, it was quite fascinating. All of the big 3 use different rack setups, but they're basically the same on every tower.

Here's a list of MVNOs for the US:
https://en.wikipedia.org/wiki/List_o...work_operators