Texture on Top of 3D Tile

How would I go about achieving what is shown in this Video?

https://www.reddit.com/r/Unity3D/comments/7acboe/bug_wars_some_early_gameplay_give_your_feedback/

What I’m interested in is that “Where can I walk” Indication.
Is this just a Texture that gets filled in a custom Shader?

I would like to be able to have different Textures on that.

The easiest way to do that are probably projectors (Component->Effects->Projector). Simply place an orthographic one facing downwards above your tile. Of course writing a custom shader is also possible and probably a bit faster. If you have a square grid like in the video, you can just tile a bunch of small textures over the entire board and mask them either with another texture or with vertex colors. I suppose it would be quite similar to the default terrain shader (which you can download from Unity’s official website).