Can someone explain to me why: Why it is so hot upstairs but cold down stairs. I know its have to do with air not being push but I know there more to it than just that

Heat air always rises. Its why hot air balloons float. In water, the hot water will always be on top.

Some reasons why.

*Air flow is being restricted between the furnace to the registers.

*Ductwork is not the right size or hasn't been sealed properly.

*Your furnace or air conditioning system isn't the right size for your home.

*Thermostat is only reading the temperature of the floor that is being heated/cooled.