Bsb is BuckleScript's build system. User-facing documentations are here.
This directory hosts its implementation. It reads into bsconfig.json, uses some BS/OCaml/Reason-specific logic, and generates a ninja build file then calls ninja on it. So much of the incremental build and perf work is delegated to Ninja.
There's a templates/ subdirectory. It's the thing shown when you do bsb -themes. To generate a template for the user, it basically picks the chosen template from templates/ and copy pastes it into the destined user directory while substituting some strings in those templates, like ${bsb:proj-version} in the package.jsons. The copy-pasting of folders isn't actually done naively through a call to unix cp. It's cleverly achieved through something called ocamlres. Please see more descriptions in pack-templates.sh.