I watched the youtube video and applied the terrain texture to the plain geometry, but it's strange

When I gltf loader the terrain created with blender plain with three.js, only the plane comes out, is there a solution?

It looks like you didn’t apply the modifier in Blender.

Also, you can use displacement maps directly in Threejs on a plane if it contains many vertices. No need to create your model in Blender first if you don’t really want to.
Example : DisplacementMap