Hi,
Basically every elements on the demo was an SVG file loaded into fabricjs canvas, and then use that canvas as CanvasTexture for the 3d model. By modifying the fabricjs objects, you will have the ability to design with 3d preview.
I suggest u follow my thread here Three.js + Fabric.js but with mouse and touch raycast - #43 by Andrew_Bilous there u can read there’s a link to prisoner849 post where he shared the foundation of what i’ve built.