Similarity between all gases
The way to get a handle on why gases are all similar is to think about the relationships between how the particles of the gas behave and the macro properties temperature and pressure.

The temperature is a measure of the average kinetic energy of the particles. The pressure is a consequence of the particles of the gas bouncing off the walls of the vessel the gas is contained in or the surfaces in contact with the gas. In a gas made of heavy particles the average speed of the particles will be much lower than for a gas made of light particles. When you do the calculations about the effect at a surface (where the pressure is a result of the particles bouncing elastically off the surface) the effects of the mass cancel out because the heavier particles travel more slowly at the same temperature. Overall this means that, for an ideal gas, the mass of the particles doesn't matter for the pressure and all gases behave the same regardless of the particles they are made from.