Outline shader using shader graph tutorial for free

An Introduction to Blender Shader Nodes for Beginners

Shader Forge - A visual, node-based shader editor

Free HLSL Shader Tutorial Videos

Tutorial: Physically-Based Cel Shading In Unreal Engine 5

Easiest and very - very adjustable Cel Shader with Curve Atlas that works with Colored Lights, Translucency, Metallic, Refraction and doesn't require to drop off skybox. Use as many bands as You want. Link in comments

ShaderGraph: Color - Unity Learn

Blender: Introduction to Shader Editor - STYLY

Shader Forge - A visual, node-based shader editor