How to use ground check with procedural gen

I was trying to make a Minecraft clone using b3agz’s Minecraft tutorial, I tried using a custom character but it uses a ground mask, since there is no chunk prefab I can’t assign the layer manually. I tried a couple of things but it won’t work.

Terrain gen:Make Minecraft in Unity 3D Tutorial - 05 - Basic Terrain - YouTube
Player:Make Minecraft in Unity 3D Tutorial - 06 - Character Controller - YouTube

cast a ray downwards, find hit point