The archive.is links work but whenever I visit archive.ph I see a welcome to nginx page. Anyone know why?

Because that’s what is configured as the “default” page to show when somebody goes straight to archive.ph. When you go to archive.ph/someurl the server then serves you a page that corresponds to that url (someurl in this example). When you go to YouTube.com/somerandomstring it takes you directly to the video. But if you just go to YouTube.com you get a bunch of “random” videos as the home page is configured to show that (grossly simplifying )