I’ve seen some original textures and I don’t understand how are they created. They are looks like it’s simple hdr image, but warped in some special way
There are several different ways to go about making textures for a sky sphere. It’s been quite a long time since I’ve done it myself but there are plenty of tutorials from users if you search YouTube or Google that will give you an idea of how to do it.
In the engine, if you look at the material setup for the Sky Sphere (M_Sky_Panning_Clouds) you’ll see how that is setup and it’s different than this a basic one.
This video is a users showing different techniques used to create different types of sky domes or sky boxes. Perhaps this will be helpful for you.
If you have any questions feel free to let me know and I’ll give you a hand!