This documentation is not using the current rendering mechanism and will be deleted by December 31st, 2020. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
The Calendar Base extension is designed to be a flexible framework that other extensions can plug into to display calendar data in a common format and location, without in any way limiting the functionality available for these extensions. To this end, the overall architecture relies heavily on the Model-View-Controller (MVC) pattern to separate the layout from the data and on TYPO3 Services to provide pluggable models and views.