The US extended its claims on the ocean floor by an area twice the size of California, securing rights to potentially resource-rich seabeds at a time when Washington is ramping up efforts to safeguard supplies of minerals key to future technologies.
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
China attempts to maintain their territorial claims in the South China Sea after the Philippines and Vietnam conduct island-building operations there in the early-00s: CHINA BAD!!!
America expanding their territorial claims in the Pacific because they want more natural resources: Yeah, this is fine and reasonable and democratic.
Canada, who gets no say in the matter because we're entirely reliant on the US for trade and defence: fuck, eh?
One of these claims is compatible with UNCLOS, the other absolutely is not. The US pulls plenty of international dickery, including not ratifying UNCLOS, but this claim fits within that existing international law just fine