Each folder in this repository contains a separate sample designed to demonstrate some of Customizer capabilities. You can try them out in your organization by copy/pasting one or more of the sample JSON files into the Application Registry in your tenant org (requires Org Administrator access). For experimental purposes, you can restrict the user audience by using some of the matching criteria explained here in the online documentation - e.g. if you just want to see the samples yourself and not expose them to all users.
To modify these customizations based on these samples you can clone this repository or copy content into a repository of your own. You can then share your repository back to this site so that it is available to Customizer in the cloud. More information on how to do this can be found here.
Any queries or problems should be logged as Issues in this repository.