Hi there,
1. this generally happens if you have made changes to the content or have imported a site, then imported another site without ‘cleaning’ out the old one first.
If the site includes any content of yours, that you wish to keep then it will be case of manually deleting, posts, pages, elements and resetting GP Settings.
If there is no content you want to keep then simply install, enable and action this plugin:
https://en-gb.wordpress.org/plugins/wp-reset/
This will put the site back to a totally clean install.
2. It varies from one site to another. The latest site you have imported is using a Header Element to add a custom image and text background:
https://docs.generatepress.com/article/header-element-overview/