You can’t create shaders in Roblox Studio, so this effect isn’t real cel shading. It’s just a bug feature that causes something similar to what a normal cel shader would do.
You can create a 3D model that looks somewhat like cel shading using reversed normals (and some other stuff) in blender, I assume that’s what you’re talking about.
Edit:
The video above is real cel shading. Visually it looks pretty similar.