Noticed with the official maps released with the editor that each building is a prefab that can be edited indervidualy by using the Enter Prefab command at which point the building becomes indervidual components/brushes so u can edit it.

How can i make a building that i have created work in this way? So every brush / model the building contains becomes a prefab that i can Enter Prefab to edit it?

Copy and paste your building into a new map, and move it close to the center (0,0,0). Save this as a .map somewhere, and you can load it as a prefab in other maps

Thank You :)