"Find an Item" box: To hide this, go up to the Gear icon at the top of the page, and choose "Edit Page" to put the page into edit mode. Once in edit mode, the upper-right-hand corner of the List will have a little drop-down arrow. When you click that arrow, you can choose the option to "Edit Web Part".
This will open the web part properties pane on the ...
you will need to start getting acquainted with the terms and what they mean
this is the navigation for the top area of your site.
your subsites can inherit the navigation from the parent site and it will look the same, while being managed at the top site
the current navigation, aka quick launch navigation, displays ...
By default the mega menu does not show the subsites.
But you can add any links you want to show your subsites under their root sites.
The mega menu is displayed on the root site its subsites (as long as the root site is associated to the Hub)