Set up your machine

Don't touch the system Ruby

Your Mac already has a ruby command — try ruby -v in a terminal right now. That's Apple's system Ruby, and it's ancient (macOS has shipped Ruby 2.6 for years, while Ruby is at 3.4 today). It exists only so some old macOS internals keep working. Never gem install into it, never rely on it for real work — you're about to install a modern, separate Ruby that takes over for everything you actually run.