Of a mini-split? https://www.fujitsugeneral.com/us/residential/what-is-a-mini...

It's the the large horizontal vents protruding from the wall you see in pretty much every low-rise (2-3 floors) residential buildings ("apaato"), or if you go to the back of the building you can see each flat's external unit, generally wall-mounted next to or below a window.

They're almost all reversible heat pumps, though their heating capacity tends to be lower than their cooling capacity.

Note that mini splits can also use a more discrete slim duct (as the link notes), but in my experience the wall mount is by far the most common, as that's way simpler and more convenient for multi-unit buildings.

So that's what those thin units are in Hong Kong pics. I just figured they were regular AC.

These are called ductless air conditioners in the U.S.