Duende IdentityServer is a protocol engine and framework and does not include any UI.
When it comes to UIs around your authentication workflow, we designed this to be an extensibility point, and our Quickstart UI gives you a very good starting point for your own implementation.
When it comes to Admin/configuration UI capabilities, we always deliberately excluded that from the product. There are many different scenarios and approaches how you might want to integrate configuration/administration of IdentityServer in your system.
One aspect to consider is if you really want an admin to configure your production system manually. The other decision to make is how many of the protocol, client and resource options you want to expose to your admin (which means they need to understand those options). Very often you only need a subset.
If you want a general purpose Admin UI (forms over data style) - there are a couple of options.
There is of course Admin UI from RockSolidKnowledge which is a very mature, full featured and fully supported commercial product.
On the FOSS side of the world, there are the admin UIs from Jan Škoruba and Olivier Lefebvre!
To demonstrate that it is really not that much work to create specialized Admin UI functionality yourself, our new EntityFramework based template has some very simple UI to manage clients and scopes. You can use that as a starting point.
Sneak peak: IdentityServer Configuration API
All above approaches have the issue that IdentityServer itself does not have a full read/write configuration store abstraction, which means they need to work against EF directly.
This is something we want to fix. We are planning to add both configuration abstractions as well as APIs. This will make it very easy to create any kind of admin/config functionality like UIs, CLIs, automated setups via CI pipelines, dynamic client registration etc.