External services
Not everything comes from Awaiten. When you run under ASP.NET Core or the generic host, some services live in the host's provider. The payment gateway, the framework logger, and the configuration are wired there. External services let an Awaiten graph pull those in.
Generic variance
C# generic interfaces can be covariant (out T) or contravariant (in T). Awaiten honors that when it resolves. A handler registered for a base event type can satisfy a request for a derived one, the same way the compiler would allow the assignment.