Comment by neighbour
True but if you're referring to the fact that you can play Xbox and Xbox 360 games on newer hardware, I believe Microsoft has a team that has to individually patch these games to work for newer hardware.
Sony does something similar I believe with their new Classics Catalogue as part of their most premium PS Plus tier.
Yeah I remember the Xbox 360 being hit and miss with backwards compatibility - their FAQs said that most of the time the people working on it had to just look at the raw assembly of games they were trying to get running to figure out what went wrong.