This is a premium module, available with Astra Pro.
Custom Layouts is a very powerful module offered in Astra Pro. With this module, you can create totally custom header, footer, 404 page as well as as custom content or code on various hook locations.
Now you will see a new Custom Layouts menu under the Appearance tab:
Once you add a new layout, you will see options as below:
Select the option from the dropdown, where the custom layout will be appended. Please check dedicated articles on the links below for available with each layout: