Or do I need to set up Sessions in ASP.NET and get the user ID and pass it around manually like... ?
public async Task<IActionResult> Login(LoginViewModel model, string returnUrl = null)
{
ViewData["ReturnUrl"] = returnUrl;
//Save UserId in session
HttpContext.Session.SetString("UserIdKey", "123");
ViewData["UserId"] = HttpContext.Session.GetString("UserIdKey");
....