The majority of this sort of thing is usually platform ports, these have a long history of being outsourced. Bug fixing will tend to involve at least a subset o

The majority of this sort of thing is usually platform ports, these have a long history of being outsourced. Bug fixing will tend to involve at least a subset of the original team as shipping code in a game can be err... interesting by the end. Otherwise it's mostly bringing in specialists to write specific things that will then be maintained by the original team. With the rise of games as a service it's also becoming more common to outsource the development of the game and develop the platform layer in house. Or visa versa depending on the expertise of the studio.