Unless there’s been a recent change, it would be Verizon. The new T-Mobile network service is, as far as I know, still in beta testing.
Yes, eSIM is available on Mobi. Pixels as old as the 3a series are eSIM capable.
If you’d prefer not to wait for Mobi’s forthcoming T-Mobile network service, I also have household lines on Tello. Tello uses T-Mobile’s network and eSIM is available. Like Mobi, I’ve found Tello’s customer service to be excellent as have others: