The pipes and ceiling tiles in this image are tile meshes, which you can extrude to create visually interesting details without much effort. The walls also use Hotspot materials that take the edges of the geometry into account and automatically align the floor/edge/ceiling parts while tiling seamlessly in the middle. Really easy to make your amateur map look not-shit compared to Unity or Source 1.

If anyone else wants to get started with Source 2 there's a really good bunch of tutorials on this guy's channel:
https://www.youtube.com/playlist?list=PLFo8L3yruNpFjn2_SsCiQyxmnZib8QbNB