Often when viewing a website on a mobile device you'll get a notification that a mobile app is available, or if you already have the app downloaded you'll have the option to open the URL in the app.
Is there a native way to do this or are these notifications always custom?
When users click on a link or download a file and the suggestion pops up to open the respective app, that's called deep linking. iOS labels it as Universal Links and Android uses App Links.