Universal Links
Universal Links are Apple's way of deep linking with ordinary https URLs: a link on a domain associated with your app opens the app when it's installed, and the web page when it isn't.
The association is two-sided. Your app lists the domain in its Associated Domains entitlement (applinks:yourdomain.com), and the domain serves an apple-app-site-association file naming your app's Team ID and bundle ID. iOS checks the pairing ahead of time, through Apple's CDN.
Universal Links open the app only when the tap comes from somewhere that hands the URL to iOS — Messages, Notes, Mail. Typing a URL into Safari never opens an app, and many social apps open links in their own in-app browser, where iOS never sees them. Check your files with the free Universal Links validator.