In the last two parts of the Sitecore Identity series, I described the basics and an understanding of the architecture and how IdentityServer4 is embedded and used in Sitecore 9.1+, the second part was a demo for adding a web client that authenticates itself against the Sitecore Identity (meaning that a custom web application uses Sitecore as the login method think like Login using Facebook or any other Identity Provider).
The third post of the series is a bit new, as there aren’t any samples (that I could find). I will show how to add a native mobile app client (iOS for this post but Android works the exact same).
This will enable you first to authenticate against the Sitecore Identity, then secondly (later posts coming) consume protected APIs (whether Sitecore native or custom ones).