Is there any no air proof design for the wings? Or are some of the parts are equivalent to outside pressure and some have gauge pressures?
Are some parts airtight and others not?
Structures on aircraft will typically not be air tight. Even pressurized fuselages with outflow valves closed will leak some amount of air that needs to be replaced. The pressurization puts a lot of strain into the skin and supporting structure. Fuel tanks, which have to be liquid tight, have vents to allow air in and out as the fuel level changes, and to allow overflow of fuel.
There would be no purpose to making parts like wings air tight, and as the aircraft climbs and descends the pressure differential would put a lot of force into the parts. The parts will be mostly sealed to prevent things like rain from getting in but will also have drains to allow any liquids to escape and for pressure to equalize.