See the post above yours; they do both. I'm in a building where Sonic is offered but it's through AT&T. I went with AT&T since the price was the same, would still need to rent AT&T's stupid PON device (which I bypass anyway), and Sonic mentioned that AT&T provided poor customer service for Sonic customers.

I'd love it if the fiber in the building was public so I could just pay 1/2 what I pay AT&T for Sonic instead.

Yes, I'm a Sonic fiber customer on AT&T fiber. AT&T terminated fiber to the house, and my bits go over their network (with all the negatives that implies), but my service is uncapped and my bill comes from Sonic. It's more than $40 though.