Hi I have tried to make some custom architecture meshes but when I activate the snow shader the snow looks just like one single plain white or gray thing , very bad .. how does it work to make it properly?

Make sure you're using the right show shader. There are some for farm buildings, some for stone, some for objects. Also make sure you have depth biasing on.

The shaders are simply a direction application of a texture, so it may not automatically look good on all objects.

Though I'm not sure what you mean by single sheet of gray, can you provide a picture?

what is e depth biasing on. ?

On the top toolbar, click View > (Toggle, I think?) Depth Biasing. It will then properly display shaders and overlapping textures in the CK, as it really would ingame. Your snow shader should now look fine.

No is not tha , The other shaders look good on other objects and are on ... I applied the same texture of mountains but it looks different on mountain than on my mesh , and is not that mountains do have any particular more bumpy 3d model ...