The way it’s usually done is :
- block the level out in BSP, test size/scale/distances.
- test things like basic lighting, pickups and game mechanics
- Add base static meshes and start of the beautification of the level and material passes (this is a iterative process).
- Finalize mesh placement and materials, add finalized lighting.
- Add particles, atmospherics and detail lighting.
The entire level can be done in BSP or meshes, take a look at some of the demonstration levels/content examples as to how they are done. The shooter game is a good example of this, with both an entire level made of static mesh and also one which is 50/50 mesh/bsp.