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

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.

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