What's the difference between post-process in the project settings, and post-process volumes?

You usually want a PP Volume in your scene. It has all the relevant settings in it and you have more control over it’s flexbility. You can also have various post volumes in your scene with differnt effects. When you set it to “unbound” it will affect the entire world.

See the Content Examples project from Epic for a detailed walkthrough, also the documentation is of big help for understanding volumes: Post Process Effects in Unreal Engine | Unreal Engine 5.3 Documentation