Full reference of functions in Extensions Functions
Plugins are placed in the plugin folder. You can find out where it is in Trndi’s settings:

Trndi can be built without plugin support aswell; if so, the box is grayed-out:
Official versions of Trndi support extensions on Windows and Linux (amd64/x64). Due to limitations in the engine Trndi uses, extensions are not supported on arm(64) platforms such as RaspberryPi and macOS.

To create, or install, a plugin - create/move a .js file in/to the plugin folder. It will automatically load on the next start of Trndi.
If you are using multiple users in Trndi, plugins will run independently, “sandboxed”, per user.
See the full reference of functions in Extensions Functions
To have your extensions show their name and copyright, add a header at the very start:
/* My Extension
(c) My Name*/
If no /* is present, the plugin will not show any info. If your first row is another comment it might be shown as copyright.