Identity caches encrypted user data within an authentication cookie.
The following is how to get to the user name after a successful authentication.
var username = User.Identity.Name;
Identity has the GetUserId() extension which fetches the user Id.
Do not add sensitive data like the user Id in the URL. When using an Id route parameter always make sure the current user (User.Identity.Name) can view/edit the data. This is typically a database design.