How do I debug standard D365FO code?

Start from the business action you can reproduce, set a breakpoint at a known entry point, and follow the call stack.

Good places to start:

• Form button clicked()
• Controller/service entry point
• Table insert(), update(), validateWrite(), or modifiedField()
• Posting class method
• The code path adding the Infolog error

Do not try to understand an entire posting framework before you have even hit one breakpoint.

Hit a known point first, reproduce the issue, then follow the actual call stack. That tells you what ran instead of what you think probably ran.