Not a silly question at all. Firstly, heat pumps do not need to extract heat from the ground, they can also extract it from the air or a body of water. The heat

Not a silly question at all. Firstly, heat pumps do not need to extract heat from the ground, they can also extract it from the air or a body of water. The heat moved from outside to inside a building will always leak out eventually, returning to the environment. In the case of air heat pumps this means there is very little change of local air temperature over any time scale. You are right that extracting too much heat from the ground will make the ground around the exchanger too cold, which will make the process less efficient and have negative environmental effects, which means it is illegal in many places. Smaller systems like for homes are usually very shallow and will regain enough heat from the air and sun over the course of the year. Larger systems can be problematic. The common solution to this is to use the ground as a thermal battery (or install a thermal battery in the ground, using it for insulation). Then you can use your heat pump to extract heat in the cold season and in the warm season you can cool the building and move that heat back underground. This has the extra benefit of being even more efficient, since you are basically just trading heat and cold from season to season.