UNCLOS has rules defining where the continental shelf boundary can be considered to extend to, so it's probably wherever that line is (with carveouts for where it meets Russia and Canada's equivalent claims). This type of claim is only for the sea bed, not the water column, so it hasn't been economically viable to exploit in the past. That might be changing with the increased importance of rare earth metals

According to the State Dept announcement, the new lines were determined by the Extended Continental Shelf Task Force, which is a cross-agency task force representing 14 different federal agencies. More likely, the expanded areas have a variety of resources that would be of interest to US industries.