Probuilder is a powerful tool in Unity for making buildings and other smooth edged structures. This process can be much quicker then importing models from Blender and also allows it to be modified in engine.
If doing this tutorial Read the pinned comment!
How to create a door
Creating Caves using Probuilder and Polybrush
Progrids is an extension to ProBuilder
https://docs.unity3d.com/Packages/com.unity.progrids@3.0/manual/index.html
https://unity3d.com/unity/features/worldbuilding/probuilder