In EU the depopulation of AI outbreaks is under control of EU Regulation 1099/2009, which states the different methods allowed for the different species. To answer in details we need more than a few words, but I try to summarize. In general with big flocks of commercial poultry farms we use to gas animals, with CO2 or N2. We can do this operation in the whole house or in dedicated gas chambers (containers) outside the house. I consider that the loading of birds to this containers (even if the container is usually in front or very close to the house where infected birds are) is a very critical point for the spread of the virus, so for this reason I prefer the whole house gasing. In this case we have only to remove carrcasses at the end of the operations from inside to the container but the movements of tractors and people from inside to outside are less than that we must do to gas birds directly on the container and so the rysk of spread of the virus outside is probably lower.
In addition there are other reason to make me prefer the whole house gasing such as less stress for birds, less stress for operators, easier control of the proper management of the operations.