Need help. I can’t understand how to unwrap this cave
assuming that’s a seamless texture, you can just press “U” and then “reset” so every single quad takes up the whole texture.
do you understand how the faces respond to the 2d mapping?
U -> reset - doesn’t work. It looks bad
You will have to play with seams.
Start by watching UV Unwrapping Fundamentals https://youtu.be/Y7M-B6xnaEM?list=PLa1F2ddGya_-UvuAqHAksYnB0qL9yWDO6
 Or maybe in this case a simple box mapping will be enough, use the MagicUV addon (bundled) UVW algorithm which is a way much better than the default Cube Projection.
I suggest you make a primitive shape first, such as a cube, and go to the UV Editor with a texture that you like, and then pick a face and drag its points around the UV Map so that you “get it”, you will see immediate feedback in the view showing you how the face is changed.
Like Vince is pointing out, there are a number of tools that blender has to speed up the process of mapping, but before that I do think you better know what you are mapping/what your end goal actually is. Don’t worry, it’s actually very intuitive and I’m confident you will get it immediately once you play with a basic shape.
Select an Edge-Loop (Ctrl+LMB).
Ctrl+E -> Mark Seam
Select All (A)
U -> Unwrap
Then with the UV-Square Addon -> To Grid by Shape
Thanks a lot