Emacs

You might get pretty far just by explaining that all emacs “plugins” use the same language as the editor, and then explaining the C-h family of keybinds that let you see documentation for every variable and function.

Then open one of those help pages for a package to show it and highlight the part where it tells you what file the function comes from, even for 3rd party packages. Then highlight how you can extend emacs in the same way, just writing some elisp on a file instead of needing to create an entire plugin.

A lot of emacs really needs to be discovered, but I’d imagine most developers could appreciate good documentation, the ability to see exactly what code is doing, and the option to override or extend that functionality very easily.