The logic to handle authentication redirection is located in the Authenticate middleware, located in your project's app/Http/Middleware.
You should change line 18 in this file:
https://github.com/laravel/laravel/blob/master/app/Http/Middleware/Authenticate.php#L18
To return an URL to the desired route where to redirect a guest user. By the default it redirects to a named route with name login. If you are not using the default auth scaffolding you can manually change this file for something you are using, for example:
if (! $request->expectsJson()) {
return url('/login');
}