.BSP is nothing to do with Hammer! Hammer editor edits VMF files. VMFs are TEXT files, Declarative format, that contains SHAPES. You can write VMF parser in 10 minutes in C++, if this makes sense. It’s the VBSP tool that parses the VMF and compiles it into CSG binary space partitioned format with lightmaps.

“Narbacular Drop was on the GoldSrc engine” - NOPE, it was a custom engine written by one-man! Please, read a bit about this!