Hello! I really need to know it secret. How does it render highfield layers in the Houdini viewport?
It creates a volvis_blend_masks attribute, with names of volumes. Then these masks are bind in the Material Builder and blended with the previous masks in VOP Inline nodes. But how is the result of all this shown in the viewport? There are no OpenGL tags for parameters.

The heightfield material found inside the materials folder in the HDA does indeed have GL parameters -- all the names are ogl-prefixed parms that the viewport recognizes, which are named the same as the OGL tags. The viewer interprets a parm with the same name as a tag or a parm tagged with ogl.

I see ch("../../tex1_tintr") on the top of the HDA linked to diff1_tint (adjusts the color in the shader) and
ogl_uvlayer2_tex_tint (displays color in viewport).
But how are they mixed toghter to show different colors of different heightfield layers?
I mean how mixed
ogl_uvlayer2_tex_tint
ogl_uvlayer3_tex_tint
ogl_uvlayer4_tex_tint
ogl_uvlayer5_tex_tint
etc in viewport

There is a specific GL shader for rendering heightfields (material/GL32/beauty_game_lit.frag) which uses glH_GameMaterial (found in block/GL32/glH_GameMaterial.blk) and some blending code (block/GL32/game_layer.blk). At present there is no way to customize these, short of editing these files directly.