How to add HN to home screen of iOS and make it launch fullscreen?

I think you perhaps mean a Progressive Web App https://developer.mozilla.org/enUS/docs/Web/Progressive_web_...

At the minute HN is not a PWA. There are some implementations of HN as a PWA try google :)

Would be cool if HN did this though i agree.

Not sure what you mean with launch full-screen, but you can add a shortcut to any website on your home screen using Safari. https://www.wikihow.tech/Add-a-Link-Button-to-the-Home-Scree...