That's configuring auth in .NET core. It has little to do with identity server.
.AddIdentityServerAuthentication (options => {
options.Authority = "http://localhost:5000/";
options.RequireHttpsMetadata = false;
options.ApiName = "api1";
});
This line is calling middleware provided by IdentityServer.
I googled and found this
Certified OpenID Connect Implementations – OpenID[
^]