There isn’t specific documentation for Split at this time, but we’re working on docs for all sites.
To split the menu items, you’ll need to re-add the separator menu item. Check out #5 and #6 here: https://docs.generatepress.com/article/centering-logo-navigation/#using-the-header
The hero should be found in “Appearance > Elements”.
Both of these Elements should have imported for you if you did the content import. Did you remove them maybe?
Let me know 🙂