This is specific to my old neighborhood:
It goes to a small hub, looked like a small green stantion/pillar, that connects the nearby houses. My old home's was in my backyard, at one point I saw about 6 other runs for neighbors.
Then that hub, with bigger/more cables, connects to a larger hub. This was in the middle of my neighborhood by the school, and it was a quite large green box, probably 6ft tall.
From there I didn't know where it went, but same concepts apply. That would go to an even larger hub, connecting multiple neighborhoods. Depending on your area and ISP, eventually they hit an end point your ISP manages which is probably a big building where they're "connected to the rest of the internet."
You can try Splitgage 2 for yourself. It's in an open beta right now. IMO it's fantastic and feels like peak Halo 2/3 multiplayer + portals.