Howdy Donnie,
The plugins are indeed open source. The files you get to install on your WordPress site contain the source code. Our core free plugin is even hosted publicly on github: https://github.com/moderntribe/the-events-calendar
Further, our plugins have a rich API and documentation, allowing you to tack into functions and classes via WP Filters and Hooks. Significant modifications can be made without directly modifying the plugins. This gives you the benefit of being able to easily update the plugins and get new features/improvements, usually without needing to alter your modifications.
Does that all make sense?
Cheers!
– Brook