How to do path plannin without using Navmesh or terrains by simply adding game objects as obstacles

I want to do path planning in 3D something like this video

(the source code is available here) I want to do exactly the same thing I give a start and goal point, there are so game objects in my scene say cubes, it should generate a path from start to goal avoiding the obstacles or not passing through the cubes/obstacles. How do I edit this code?

I am new to Unity and I have no idea how to get this working and have a deadline to meet.