Use a parameter from url in a controller Laravel 5.8.22

I'm trying to pass anINT from this url: myapp.build/courses/anINT (implemented through CoursesController) to $id in the Lesson_unitsController function below. I've tried a lot of solutions, but I can't seem to get it right.

public function index()
{
 $id = ?????
 $course = Course::find($id);
 return view('lesson_units.index')->with('lesson_units', $course->lesson_units);

The routes in web.php are:

Route::resource('courses', 'CoursesController');
Route::resource('lesson_units', 'Lesson_unitsController');

Do I need another line in the web.php file and can the index function take parameters? If so, how? Thanks

via Chebli Mohamed