The "Extra" blog is an additional Magento 2 module built as an exercise for the M.academy "Magento 2 Coding Kickstart" course by Mark Shust but with additional functionality. It implements "before", "after", and "around" plugins that target the base module Denal05_MacademyM2CodingKickstartBlog.
The original source code for this module of the above course can be found at https://github.com/macademy/magento-2-coding-kickstart-extra.