The key to cel-shading is to modify the lighting of models such that there is at least one hard cut between lit and unlit sections.
You might have heard cel-shading also referred to as “toon lighting” or simply a “cartoonish” style it was popularised in the early-to-mid noughties by games such as Jet Set Radio and The Legend of Zelda: The Wind Waker. But it’s certainly not the only way to light your scene - many games gain a stronger sense of identity through highly stylised aesthetics, including cel-shading. Many modern games opt for a photorealistic style, attempting to mimic the way light behaves in the real world - techniques such as raytracing take this to the next level by simulating the way light refracts through, is absorbed by, and reflects off surfaces.
There are many types of lighting model for adding character to your scene.