There are different ways:
- everything is a BSP brush (not recommended)
- everything is a static mesh -> converted from a BSP brush
- make a mixture -> one part BSP another part with static mesh
- everything is a static mesh made in a 3d program
“BSPs are a great tool for blocking out a level, but it is not recommended that they be used in a final product. You will also find static meshes easier to manipulate with blueprints. I hope that this clarifies.”