Construction of golf course is a very long and complex process in real. It requires some great architectural skills and planning to just put the model on paper. Designing the same course for game although isn’t as difficult but is still a very complex process as one has to make it as close to the real thing as possible.
Golf courses basically have greens, fairway, rough, bunkers and water hazards. And one has to design each of these things to make it look exactly same as the real one. In addition to these there are various objects such as rock, trees, and bushes etc which require detailed designing and also the placement should be as per the real golf course. There are so many trees involved in golf course with each of different types and hence just creating the trees is a time consuming process.
Most of this work is done by designers of golf simulators companies as they are in constant need of new golf courses. As far as I know none of the simulators companies have more than 100 golf courses in their list as designing a new course in not cake walk it requires efforts and time. Hence you will always find that the list grows slowly. These courses are made using golf course creation engine which allows creating various parts of golf courses and once the course is designed, the objects such as trees and rocks are placed on it.
Although, I have describe the process in few words but trust me it is much more tedious and might require many more things which I am not aware of. Various reference using maps and images need to be collected before even starting to design a golf course which in itself is a time consuming job.