There is nothing to share about diagnostic codes. Since 1996, fault code numbers and definitions have been standardized–a P0401 means insufficient EGR flow for every make, model, and engine across the board. That’s federally mandated.
The lawsuits and pending legislation in many states (“Right to Repair”) are frivolous and in my opinion stupid. With very few exceptions, everything your corner garage needs to fix your car is available to them. Hardware, software, diagnostic and testing equipment is all available to anyone who wants to buy. But people want these things for free.
AutoZone, etc. don’t do diagnostics. They read fault codes and give a list of possible faulty parts. There may be half a dozen things causing a given code, but all those parts are easily tested and verified as good or bad if you have the correct equipment and service information.