Cookie Authentication: How to access a returning user - asp.net-core

Question

I have studied a couple examples of ASP.Net Core 2.0 Cookie Authentication.
However, I still don't see the code that checks for a returning user via a cookie.
Please note that I'm not a web developer. As a result, I realize that my question may appear foolish.
As I understand, the browser sends the cookie to the server per client request. However, I just don't see the server logic for this in the examples that I've studied. Hence, I only see logic for logging in the user the very first time based a username and password that's explicitly passed in.
Expectation:
I expected the server to explicitly check if a cookie exists when requesting the index page of the website. If a cookie does exist, I would expect to see some logic to address a user that's already logged in.
Giraffe example
Tutorial example
Here's my code:
let loginHandler =
fun (next : HttpFunc) (ctx : HttpContext) ->
Tasks.Task.Run(fun _ -> StackOverflow.CachedTags.Instance() |> ignore) |> ignore
task {
let! data = ctx.BindJsonAsync<LogInRequest>()
let email = data.Email.ToLower()
if authenticate email data.Password
then match login email with
| Some provider ->
let claims = [ Claim(ClaimTypes.Name, email) ]
let identity = ClaimsIdentity(claims, authScheme)
let user = ClaimsPrincipal(identity)
do! ctx.SignInAsync(authScheme, user)
return! json provider next ctx
| None -> return! (setStatusCode 400 >=> json "Invalid login") next ctx
else return! (setStatusCode 400 >=> json "Invalid login") next ctx
}

Answer 1

I'm looking at the giraffe example.
The statements services.AddAuthentication(authScheme) and services.AddCookie(cookieAuth) will add various services to the services collection, making them available for injection.
The statement app.UseAuthentication() adds services to the middleware pipeline. A middleware service is something that runs on every request.
The service that gets added by UseAuthentication is called AuthenticationMiddleware. As you can see this middleware relies on an injected IAuthenticationSchemeProvider (provided by your call to AddAuthentication) which in turn (skipping a few steps here) relies on a service called CookieAuthenticationhandler (provided by your call to AddCookie) which actually does the work of authenticating via cookie. When it is done it sets the logged in user onto the HttpContext which you can reference from your controllers if you need to.