For personal project there will not be a TDD so coder not willing to share their existing codes. I rather showcase my skills in a new development than say already done one.
There is a real treasure trove of things to talk about.
Questions like: - Why is it like Y? - What is good and bad about Y? - Why do you think X would be better? - Why did you end up doing Y instead of X? - How would you approach changing it to X?
Discussions like these on both a macro and micro level is very valuable.