If you look at any sample, you will see that “$” is just the GraphObject.make static function: GraphObject | GoJS API
Basically the first argument must be either a class or a string builder name defined by GraphObject | GoJS API. The latter lists all of the pre-defined builders, but you can define your own.
The rest of the arguments are initializers for the object being made.
For Panels, that may include other GraphObjects, which are added to the panel by calling Panel.add.