I was wrong… the terminal itself (the “optical network terminal”, or ONT) IS the modem. They come in both external (mounted on an exterior wall of the house) and internal (inside your room) varieties. AT&T gave me a small internal one, the middle one here:
Other providers may differ. And more complex ONTs (like for a new construction home where everything is served via fiber) may include connections for whole-house coax networking, landline over VOIP, cable TV, etc. There are also bigger units that serve apartment complexes and businesses.
Windstream’s seems to work a similar way: https://www.windstream.com/blog/how-does-fiber-internet-work
It sounds like their ONT just gives you a standard ethernet port to connect to a router of your choice, but it might be worth calling them to verify. The Windstream might look something like this:
(judging from random internet threads that mention this)