Some level designers will use BSP to block out a level without actually doing any detail modeling. The vast majority of the time you’ll want to model everything in a 3D program like Blender, Maya, or 3ds Max and then export to UE4.
You can design things so that you can have a set of meshes that you would export to UE4 and then you would use them to add the detail to the map, or you could design the level entirely in your 3D program.