That depends on what you want to do. OBDII diagnostics is legally mandated by law and so not hard to do across all cars. It is minimal information, but enough for most things. If you want anything else it different for each car (not just each manufacture, sometimes sometimes the same car model will have more than one variant).

The information you need isn't impossible to get: the manufactures make this available to third party scan tools, but they assume anyone wanting this information considers paying $100,000 a nominal cost and won't question it. In a previous job (I left in 2010) I had access to this information, but we still spent a lot of time reverse engineering things because the documents were too often wrong.