The PasswordSignInAsync creates an authentication cookie which is returned to the browser. On the next and subsequent requests the browser sends the cookie which is used to populate the user's (User) principal. Also, if the apiSigIn is an API, API does not use cookies by default. You did not share the configuration so we have no way of knowing how the application is setup.
I recommend creating a Razor Pages or MVC application and scaffolding Identity which has sample code you can review. You'll notice that after a successful sign in, the browser is redirected.
Also see the official documentation which illustrates how to use Identity in an SPA.