While I condone editing a previously completed map, and doubt that the vmf you have is an official version, I will be kind and point you in the right direction.
This is a compile tool used to convert brushes into static / dynamic, or physics models.
Instructions for this program can be found at the link.
While this may not be exactly what Antimonycat uses, he is most likely converting allot of his non occluding brushes into models, and by the looks of it, with a real modeling program.
Enjoy your mapping, please don’t ask on another thread on how to use Propper, there are plenty of similar threads, and loads of online resources.
Did some searching around, the only .vmf(s) I can find of ‘rp_c18_v1’ are decompiled versions, as apparently the original .vmf was lost.
Decompiling is a horrendous process and should only be used for research, not to build off of, this would contribute to your high brush count.