Limitations based on copyrights significantly hinder the helpfulness of this site, although the design is top notch.
Either way, you as the website owner are not responsible for whatever font ends up being chosen by the browser to render the website.
No they’re not. Apple, Microsoft and even some Google systems include fonts which are commercially licensed and usable only on a device licensed for that OS. In fact Apple and Microsoft contracted the development of many fonts you see on today’s devices and the web.
Disclaimer: Not legal advice
San Francisco, Segoe, and many other OS-included fonts have commercial proprietary licenses, not permissive licenses as the GGP post claimed. Try to serve San Francisco WOFF2 files as a font-family src on your site and see how Apple’s lawyers react.