You probably won't find many resources for refactoring great examples step by step. Because you can never cover all types of examples.
The reason Martin Fowler uses a small and simple example in Refactoring is because almost every big piece of bad code is a combination of another bad smell. By learning how to recognize a specific bad smell, you can fix the code gradually.
I recommend that you check Effectively work with outdated code . This is a book that focuses on strategies for improving large legacy code. For class design, you probably want to read some books on the design pattern.
Most importantly, try applying the things you learn in the book to your code.
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