I was recently chatting with 2 airline agents at a small west coast airport serviced by AA, UA and AA with an average of 13-14 daily flights to 8 destinations.
Turns out, all flights (besides for 1) are operated by SkyWest. UA and AA contract ground operations to SkyWest as well, while AA contracts ground operations to Piedmont.
Piedmont has a staff of 5 at the airport. Seems like they are responsible for check in, GA, loading and unloading luggage, and cleaning aircraft after flights.
The big news at the airport is that AA Mainline is coming to the airport in October, although I think ground ops will still be Piedmont. I think it'll replace the 1 daily flight currently operated by Envoy Air.
It also seems like 1 of the Alaska SkyWest flights will be changing soon to Alaska Horizon.