AI models to understand legacy code without code generation

I'm looking for AI-based tools that can help me understand a system's legacy components. Ideally, I would provide the source code, and the model would generate text or diagrams that explain the code's underlying business rules as much as possible.

The broad range of existing LLM tooling seems to focus specifically on the code generation part, but I'm not interested in getting generated code. I love to code, so I would like these models to help me in the boring part of understanding legacy code before the modernization.

The only relevant article I have found so far is https://martinfowler.com/articles/legacy-modernization-gen-ai.html . However, it's based on a private tool by Thoughtworks.