I was looking into this recently.
The total stack up of pipes and tools you lower into the bore hole is called Drill String. The design of the drill string determines what depth you can drill to. It must be long enough to reach the depth, it must be also strong enough to withstand the pressure and light enough so it can be lifted from the bore hole. It often gets stuck so a large margin of strength is necessary so the string can be pulled out with brute force.
If the drill string was made of one type of pipe it would be too heavy so it generally tapers down as you go deeper. This means the string must be withdrawn several times in order to install the thinner pipes to the bottom end. The operation of withdrawing the whole string is called a trip. Tripping wastes a lot of time and is very labor intensive. All the manual handling also increases the odds of personal injury. Everything around a drill head is big hard and heavy so injuries, when they happen, are severe.
Longer pipe lenghts mean less tripping but the length of the pipe is limited by the transport requirements. So 12 meters is the standard length if the pipes are to be transported by road. Sometimes the lenghts are pre-assembled in lenghts of two to make things more efficient. This requires the derrick to be twice as high, again transport limitations.
The drill bit is the thing that does the actual job of drilling. It is a consumable and wears out alarmingly quickly and is also very expensive. Changing a drill bit requires a trip. If a trip happens for some other reason the drill bit is also changed even if not completely worn out, this saves a trip. It can be said that the cost of the operation is driven by the number of trips. Least amount of trips makes the cheapest hole.
All sorts of drill bits depending on what you are trying to drill. The bits can be powered either by rotational torque provided by a rotating collar on the drill platform which drives a heavy square pipe called a kelly, or by mud pressure. A new development in this area is the Top Drive which eliminates the need for a kelly and the associated manual handling.
The mud pressure is provided by mud pumps several hundred horsepower each. Usually there are more than one for redundancy because the wear and tear is severe.
The mud itself is an entire engineering field on its own. There are a dozen chemicals that go into the mud and a dozen pieces of equipment it passes trough each time it is circulated. Mud serves to cool the drill bit, carry out cuttings, stabilize the bore walls, reduce friction, maintain pressure to prevent collapse, provide early warning for kickback or blowout, and many other things. In general the mud is the secret ingredient that lets you cheat nature and drill deep.
All of the above factors make drilling an expensive undertaking. Maximum utility from each bore hole is desired. This means you would want to generate electricity from the heat 24/7. Only seasonal heating is not enough to justify the cost.
All of the above is the result of some superficial reading I've done. By reading quite a lot more I am hoping to be able to do cost estimations and feasibility studies in the future. Sorry for the wall of text.