Verizon and ATT just rebrand nokia ONTs and roll some of their own software that is mostly enhanced or changed encryption at L1. Can’t speak for Comcast, I only know about the other two as I’m in a smaller ISP that competes with them.

They use have L2 onts that don’t have any gateway functions, just fiber to ethernet with some extra overhead to monitor the connection between the hose and shelf.

The ONT-on-a-stick units do the same thing, just a more compact and expensive interface that doesn’t have great support, unless comcast or running all home run fibers where they can just provide a straight SFP instead of doing any optical splitting.