In AR, the digital elements are added to layer the user’s real-world or enhance it with the virtual effect.
Mixed reality creates an environment where the user can interact with the virtual objects. So instead of residing in a completely virtual world, mixed reality helps the user stay in the real world and gives the advantages of the virtual world.
Now to answer your question of how these reality techs are different from each other.
VR replaces the original building into virtually simulated graphics whereas Augmented Reality, takes the same instance of building architecture simulating the actual building, to be built in the real.
Finally, in Mixed Reality, the technology helps to hook the virtual objects and make them responsive to/in the real world and provide the appearance of virtual interactions to be authentic to the user.