OEMs only tell authorized mechanics what the codes mean. Most cars have the basic standardized OBD-II codes, and an additional much-more-useful set of codes/diagnostics that are proprietary.
They tell the third party scan tool manufactures as well. (for a "reasonable" price - where reasonable can be 6 figures) I used to work on those scan tools, and so I saw the data (I probably shouldn't talk about it - lets just say were reversed engineered their tools often).
Many mechanics will read and tell you the codes for free. Auto part stores will as well.
Many mechanics will read and tell you the codes for free. Auto part stores will as well.