A bootstrappable build (according to bootstrappable.org) is a bootstrap process where you don't use any binaries of the compiler itself to build the compiler. Unless the compiler already has a maintained source-only bootstrap, the usual way to do that is to write a smaller implementation in another language with just enough features that it can build the compiler. camlboot is an example of that. Another option is to build a chain of previous versions of the compiler that were written in other languages, before the compiler became self-hosting.

https://github.com/Ekdohibs/camlboot