SignInManager provides ASP.NET Core Identity APIs for logging users in. An AccountController receives it through dependency injection alongside UserManager and IConfiguration, supporting separate Register and Login actions. section 9.2.6
Login service in the board-game API
A Login action validates LoginDTO, finds the user by name, checks the password, and returns a JWT for a successful authentication. SignInManager belongs to this authentication boundary, although the supplied implementation excerpt does not show a direct SignInManager call. section 9.2.6

