“It’s kind of like opening the developer tools in an Electron app, except these are native macOS apps with web views parading as system UI.”
Two defaults commands unlock Safari’s inspector inside System Preferences. The Apple ID and Family Sharing panes turn out to be web pages, one of them running React. Apple fans love to sneer at Electron apps for not being native. Apple ships the same trick in its own settings and dresses it in system colors so nobody notices.