The way I see it, there are generally speaking two routes:

1. Draw the pictures yourself,
2. Write a computer program that can generate some sort of art.

Drawing requires lots of practice, as others have already pointed out. Writing a program takes skill, patience, knowledge and time, so it’s not that different really. Main difference is what happens next. If you’ve made a picture, you have a single picture. If you’ve made a picture generating application, you can have unlimited pictures. The latter option comes with some pretty harsh limitations, because the range of possible pictures can be very limited.

Here’s an example of option 2: pattern collider