Why do they put cold air returns in the floor and not the ceiling?
Because in a fluid medium such as gas the colder, denser fluid sinks and the warmer, less dense fluid rises.
This is heat transfer via convection.
In a solid medium such as dirt and stone heat penetrates via conduction (i.e., gradually downward into the ground) when the group d temperature is colder than the air temperature and escapes via the same path and heat transfer mechanism when the ground temperature is warmer than the air temperature.
Finally there is heat transfer via radiation which you can look up if interested.