The Source code of our free products (GPLv2+ license) can be found in a public GitHub repository (GitHub user account required!).
Any help in improving the Plugin is truly appreciated. If you would like to contribute in development you need to be familiar with Git(Hub), WordPress Coding Standards and modern WordPress development (i.e. Gutenberg).
You can also help translating the Plugin to your language via https://translate.wordpress.org (WordPress.org user account required!)