While helping a bit, it's difficult to learn or reverse engineering from existing type checking code because a lot of them are mundane repetitive code implementing some high level theory and algorithms. The actual code is too far remote from the theory. You really want to start from the theory and the overall picture.

The Dragon book has a chapter on type checking. It gives explanations on many topics. It has plenty of examples. It has type treatments on different areas of a language, like expression, array, struct, function, pointer, etc.

Despite being really old, its ideas and explanation on the topic are still relevant.