This is a child theme for the Twenty Twenty WordPress theme.
Based on this documentation: https://developer.wordpress.org/themes/advanced-topics/child-themes/
And this example: https://github.com/ryandaniels/wp-twentytwenty-child-tiny
Install the child theme as you install any other theme. You can copy the folder to the site using FTP, or create a zip file of the child theme folder, choosing the option to maintain folder structure, and click on Appearance > Themes > Add New to upload the zip file.