I need help for creating and using decals. For example for creating bullet hole etc…
Unfortunately, Unity doesn’t have a very good decal system built-in. You could use projectors, but those effectively draw every object that the decal is on again, once for every decal.
For small decals like bullet holes, you might have luck creating a plane with your decal texture every time a bullet hits. You can use the information from RaycastHit to orient the plane properly.
If you want a lot of simultaneous bullet holes, you might have to look into adding new bullet holes to a dynamic mesh so that they can all be drawn at once.