Learning another codebase generally involves fitting someone else's metaphors in your head. This is why devs are tempted to write their own.
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I'm intrigued to see that Google has quantified that new code is generally buggier and less secure than code that has existed in your codebase for longer: https://security.googleblog.com/2024/09/eliminating-memory-safety-vulnerabilities-Android.html
Learning software development through playing and contributing to a MUD: https://tashian.com/articles/how-i-learned-to-program/
Over a sufficiently long time horizon, all code you write is legacy code.