I realize this isn't the typical PBR workflow, and can be unusual for many to gain experience with, but if you happen to know. I'd love to learn more about the toon shader workflow/ tips and tricks.
You can use a solidify modifier and change the material in the modifier, plus some other trickery...
Lightning Boy Studio have an awesome YT playlist to make a toon shader.
It depends on your use. In a game engine you use an inverse hull technique(the solidify mid that Adrian mentioned). If you're staying in Blender then you can use freestyle(that Martin mentioned) which is a post process effect and can be used as a render pass too. Then there's the newer line art modifier for grease pencil.
Line Art Modifier is the way to go now. It's like using Freestyle, but directly in the interface. It also has the added bonus that you can bake it all down when you're done; edit it; add modifiers to it etc and generally have some awesome fun. Is there a specific look you are after? I'd be keen to do a deep dive into the lineart modifier!
