Put some menu items under more link at moderate screen sizes show all vertically on mobile drupal 9

Actually Drutopia does this well enough already:

Screenshot of a mobile viewport showing the Drutopia.org main menu open, with three parent items each with links displayed vertically.

Maybe update this pretty cool documentation page with any techniques we come up with that can be done with core Drupal plus CSS: Menu and Navigation Features | Additional features | Drupal Wiki guide on Drupal.org