Which is (almost certainly) using the ground not as an energy source but as a thermal (energy) storage/"infinite" heatsink, that works great in isolated/sparse areas where the climate annually averages around the "comfortable" indoor temperature.
Exactly.
I don't personally like to use the word "geothermal" because it's confusing and used for different energy sources. Ground-source heatpumping can tap two different sources:
1) Actual geothermal, i.e., heat flow from the molten core of the Earth (by just drilling far enough, or utilizing natural hot water sources). Actual energy source is the massive pool of molten lava, since the Earth was born.
2) Just the thermal storage capacity of the soil or bedrock pretty close to the surface, deep enough so that it follows
yearly average climate temperature. Actual energy source is the Sun.
In Northern Finland for example, where air-source heatpumping gets impractical due to significant periods of time below say -20degC air temperature, ground source heatpump installations are used; this would be typically a 150-meter deep well, maybe a bit more. Drilling cost is roughly around 10k€. AFAIK, this is deep enough that the actual geothermal (the lava thing!) shows a tiny contribution, but still it's mostly just averaging the yearly climate, i.e., solar power. Which would be something like +5degC in those areas. Way better for heatpumping than -30degC air!
Accumulating snow works like thermal insulation during winter, so the soil/bedrock temperature is tad higher than you could guesstimate from the yearly average air temperature. So the idea is, maybe the well drilled in bedrock is at +7degC when you start using it during the fall, and maybe it's down to +2degC when you are done in the spring. Then it regenerates for the next winter.
For the energy to regenerate (i.e., the soil needs to heat up again during summer), the well needs to be deep enough, or you need two in parallel, and can't just place them close to each other. Due to the required gaps, this is not THE solution for global energy problems, but it can of course be a small part of it, in sparsely populated, very cold climates - exactly like Northern Finland.
Then again, "true" geothermal works best in areas where you naturally get access to the heated water (hot springs). There, you don't even need heatpumping; temperature could be high enough for passive energy transfer, just pump the water through heat exchangers. It's obvious to utilize this resource to the fullest if you have it, but 99.9999% of people on the planet just won't.