Best way to create maps

I want to make several scenes for my game, and I am wondering whether it’s more efficient to make them out of Unity assets and gameobjects or to create the whole thing as a lump model in Blender and then add box colliders, like Overwatch.

Well, Unity is driven by GameObjects. Overwatch, on the other hand, has a custom and proprietary engine that may not be. You are stating that Overwatch is a single mesh, and that may be true (I don’t know either way), but that would likely not perform well in Unity.