This extension makes heavy use of metaprogramming.
The general build flow looks like this:

common/ + meta/ + "catalog" database -> generated/
generated/ + build/ -> sql/

common/: Things common to multiple parts of the build process
meta/: Used to build the files that go into the generated directory
"catalog" database: A database called catalog, created from https://github.com/decibel/catalog_entities/blob/master/load.sh

generated/: These files should only need to be rebuilt to incorporate new Postgres versions.
build/: Files used during normal build process

sql/: Some files here are generated from files in build/