Fallen Skybridge

To access this location, you have to go to the back alleys shown in the map below and use the fire exit stairs until you reach the rooftop with a lift.

This seems to be former Gunner outpost, overtaken by supermutants. You'll possibly encounter two legendaries here so be prepared for a big fight. There are no notable loots here aside from the two unique equipment that the legendaries may drop. There is also a door here leading to the Hub 360 building.

There's a connected ramp leading to the Ruined Skyscraper (unmarked location). For some reason, Dogmeat won't follow you inside so if you notice that he's missing, don't worry. As long as you didn't order him to stay somewhere, he'll spawn again if you're in a larger space. You'll find a supermutant inside the only room upstairs. Take the elevator to go to the lower floor where you might encounter a small battle between gunners and super mutants.

There's another elevator at the end of the corridor and the small rooms here doesn't contain much. After taking the last elevator, you'll reach another gunner outpost. One of the doors there leads outside.