[AC Question]

I can't seem to find a definitive answer on how updates work for the Nexus 6P on ATT. I'm concerned because I thought updates for my wife's unlocked Moto G would come quickly. As soon as we popped in an ATT SIM though it seems to have attached to ATT and installed an ATT build. So we were always waiting on updates.
I'm on an ATT provided Galaxy S5 active right now. I just paid off under a Next plan and ready for new phone. If I purchase a Nexus 6P and pop in the ATT SIM would I be getting updates from ATT or directly from Google?