Federated identity from another realm of the same server

There is 1 keycloak server
The server has 2 realms.
RealmB should use RealmA to be able to log in like e.g. Facebook, Google or Github does.
(RealmA becomes something like Facebook, Google, Github)
How is something like that accomplished?

Simply configure an Identity Provider of type “Keycloak OpenID Connect” in your RealmB to point to RealmA. It has to be configured the same way as it would be a remote Keycloak instance. There’s not special option like “use realm XY from this instance”.

Ok but what do I have to write in the fields in the fieldset “OpenID Connect Config”?

I was able to find out the endpoints, there’s a link in the realm definition. “OpenID Endpoint Configuration”.

Now I’m creating a client in RealmA which is private with client_id and secret.
And I’ll use those credentials in the broker configuration in RealmB.

There’s also https://github.com/dcm4che/dcm4chee-arc-light/wiki/Standalone-Keycloak-as-Identity-Provider and https://medium.com/keycloak/keycloak-as-an-identity-broker-an-identity-provider-af1b150ea94