Second, air is a gas, which means that the pressure will be evenly distributed throughout the entire aircraft.
Strictly speaking, air is a fluid. It has mass, density, and volume and cannot resist shear.
Depending on the composition of air, the density changes, this happens are you go further away from the surface.
Depending on the local air pressure, the density also changes.
Density is mass/volume, the weight of that air (mass x g) exerts a slightly higher pressure towards earth to transfer the mass of the air over the area of the surface (maybe 8.0000000000001 psi) than it does on the walls (say 8 psi).
A better example is a high pressure gas cylinder, it exerts pressure evenly on all sides of the container, however a full gas cylinder will have a higher mass than an empty one.