AppAuth-Android VS openiddict-core

Since we use OpenID Connect as Authentication Protocol you can use projects like https://github.com/openid/AppAuth-Android to integrate with ZITADEL

You mentioned Android Studio so I assume you're developing for Android - I'd start here with the caveat that I've never done any Android development so this may not be exactly what you want.

2) Should I still need library such as https://github.com/openid/AppAuth-Android ?

Openiddict is also an option. Check it out.https://github.com/openiddict/openiddict-core

To implement an OAuth server together with OpenID connect specification, I used a free and open source framework called Openiddict. You should checkout their GitHub repository at github.com/openiddict/openiddict-core. It is a nice and feature rich tool which help you get started very fast. However, comparing to alternatives like identity server, it needs more implementation rather functionalities coming out of the box.

You can use Openidict https://documentation.openiddict.com/, give it a try

Openiddict to create your own OpenID Connect server if you want to deep dive into auth https://github.com/openiddict/openiddict-core

If you do want a .NET IdP then you could look into OpenIddict but will require some hands on work, at a minimum authorisation controllers. If you're looking for something more out of the box OrchardCore's OpenId module might be better for you.

I personally prefer using Cookies with openiddict.

Also, the OAuth bit is very cool!! Do you plan to support OpenIddict it’s a free federated identity server.

Okta is a great example of an IdP, although there are many others. Auth0, Ping Federate, Amazon cognito, etc. OP also mentioned Identity Server, which is a framework for implementing your own IdP. I've also personally used an open-source library called [OpenIDDict[(https://documentation.openiddict.com/) to great success. I would highly recommend attempting to implement one of these providers as a learning exercise.