It’s either documented or it’s not. When I modify code in a way that isn’t immediately obvious by looking at the code itself plus the immediate surroundings, I write a two-or three-lines code comment. It takes me ten seconds and does a big favor to my future self. I fail to understand why this isn’t standard procedure.

To actually answer your question, do a git blame, check the commit messages and anything linked in commit messages. Do some search in the company’s internal knowledge base, architecture documents, specs, whatever you have available. Even if you don’t find a direct answer, understanding some more context might eventually lead to one.

If you have no documentation at all anywhere, then you have to analyze the code yourself. It’s part of your job. When you’re done be sure to document your findings.