if you could tell me how to do that on Twenty Twenty-One theme.

Thanks

You can also use a navigation block. Inside the Navigation Block, click Add Block (+) to add menu items. To create a submenu, add a Page Link or Custom Link, then click on the menu item you want to serve as the parent.You should see a small submenu icon (a down arrow) that allows you to add nested items.Add submenu items directly beneath the parent item by dragging them slightly to the right or selecting the Add Submenu option.