How to make a canvas with scribbled text?

I've seen that a library called FabricJS has a feature that lets you insert text on the canvas and edit it, not through a input[type=text] and yes within the canvas itself.

Is there any special functionality of canvas itself that allows this? or everything needs to be done "on hand"?

I'd like to know what you need to know to do something like this, implementing "scratch."