ASP.NET Core Identity is very good, though I prefer not using the default UI package because a) there's no easy way to disable any of the built-in pages, and b) I customize the UI so much that I might as well maintain my own code.

There are several IServiceCollection extension methods to configure Identity:

• AddDefaultIdentity<TUser>() used by Identity UI.
• AddIdentity<TUser, TRole>() used by IdentityServer4, which also calls