Is there any way to make tunnels in landscapes without meshes and billboard mountains?

I made some cave meshes but I forgot to rememeber that with backface culling they are invisible when placed in a dugout landscape.

Is there anyway I can make a “bridge” of landscape thats a certain thickness to cover this up so that I can still do regular landscape stuff like foilage paint, procedural foilage sculpt etc. , or since landscapes are heightmap based will I need some cover up meshes/make the meshes have a normal facing both ways?

Secondly thinsg like the mountains in the disance in the kite-demo for example that you could never reach, are these billboards? I never found any billboards in the kitedemo assets or did alot of thelandscape in the kitedemo involve C++?