How user is logged in after registeration automatically in laravel anyone ?

You can use laravel/ui package or you can get the logged in user after registration like the following.

public function register(Request $request)
{
    $this->validator($request->all())->validate();
    event(new Registered($user = $this->create($request->all())));
    $this->guard()->login($user);

    Auth::loginUsingId($user->id);

    if ($response = $this->registered($request, $user)) {
        return $response;
    }

    return $request->wantsJson()
        ? new JsonResponse([], 201)
        : redirect($this->redirectPath());
}