Usually a mix of both, depends heavily on where you are, in my country security is mandated by government but the exact procedures are left to the individual ai

Usually a mix of both, depends heavily on where you are, in my country security is mandated by government but the exact procedures are left to the individual airports and evaluated by government agencies. IIRC the US has the TSA but Airports can outsource to other third parties if the TSA permits it. I don't see how that question is really relevant considering your bag will be atleast xrayed by one of the two at some point.