How can I use the shortcodes in my PHP code?

This can be done with do_shortcode() WordPress function, e.g.:

echo do_shortcode( '[countdown date_to="2021/01/01 12:00"]' );

How can I contribute?

Visit Awesome Shortcodes plugin GitHub repository.

Where can I download the plugin?

Plugin can be downloaded directly from wordpress.org repository, or through plugin home page.

I’ve installed the plugin, but none of the shortcodes are working.

To save your server resources, after installation all shortcodes are disabled by default, so you could enable only those that you will be using. To enable the shortcodes, go to Settings > Awesome Shortcodes > Shortcodes and enable needed shortcodes with checkboxes.