If you logged in to HN using Safari on a Mac the private-key (a.k.a "password") got chucked into your keychain as part of the account creation flow and is synced across all your iCloud devices.
So on your phone when visiting the HN login page you'd just be prompted for a fingerprint by TouchID and in you go. Actually quite seamless. This would be what 90%+ of users experience as normal people don't fiddle with defaults.
I don't use Windows but they have some sort of iCloud Passwords thing for Windows now too apparently. Just dipping their toe into slowly making it cross platform.
It becomes less seamless and more of a hassle when you are using multiple keychains or 3rd party apps which probably a lot of people here are. What I described is that case, when you have both an Android phone and an iPhone and they are completely sequestered from each other (maybe personal and work?).