We have the server set up and configured to return certain paths under /.well-known/apple-app-site-association. When we publish our app, the "associatedDomains" key in our app.json instructs iOS to give us a chance to open links in app.
That all, more or less, works. What we can't figure out is how to productively test universal links during local development. The expo app won't have our app.json and its a horrendous flow to make an independent build for this purpose (we lose hot reloading and frankly it just doesn't seem to work regardless)
This wouldn't be the end of the world if our product didn't require opening a link in email to log in, but it does and that requirement isn't going anywhere.