Custom URI schemes were the original form of deep linking for mobile apps. They are like creating a “private internet” for your app, with links that look like myapp://path/to/content. The advantage of custom URI schemes is they are easy to set up and most apps already have one. The disadvantage is a user’s device only knows about this “private internet” if the corresponding app is already installed, and there is no graceful fallback option by default.
This is still the primary approach to deep linking on Android, but Apple began blocking this approach on iOS in 2015 with the release of Universal Links.