Comment by geofft

Emacs somewhat famously uses "unexec" in its build process, you build a skeletal Emacs in C (mostly the Emacs Lisp implementation), run it, load and compile and process a bunch of Lisp that implements the editor itself, and dump the resulting process memory back out to disk. The result of this eldritch process is the final emacs binary. When you exec emacs, you get an environment that consists of the editor code ready to go.