If you really want to deep dive into extensions, check out the source code of all extensions you can get your hands on. Some extensions are on GitHub, others can be found on people's websites. Open up the mbrext file and dig through all files.

You can also check out most of Mobirise's extensions. Install them from within Mobirise, then browse to the addons folder on your system.

On Windows:
c:\Users\username\AppData\Local\Mobirise.com\Mobirise\email\addons

On Mac:
/Users/username/Library/Application Support/Mobirise.com/Mobirise/email/addons

Each folder in the addons directory is an extension or theme. Check out all the files you can find, open them using your favorite text editor (such as Visual Studio Code).