How do I find where an Infolog error is coming from in D365FO?

Start with the exact message.

If it is a label, search for the label ID or the label text in Visual Studio. That often takes you directly to the error(), warning(), or checkFailed() call.

If searching does not get you there, reproduce the issue under the debugger and inspect the call stack when the message is added.

The call stack is the important part because it tells you how the application reached the error in your scenario.

References show where code can run. The call stack shows what did run.