They fill the barn with suffocating foam, if the chickens are lucky.
If the chickens arent lucky, they just turn off the ventillation and close all the hatches. In full sun, the barn heats up to beyond lethal levels and the chickens die slowly of heatstroke.
Many chicken barns have massive euthenasia systems built in, with huge tanks of nitrogen or CO2, and a foam generating sprinkler.
In any case of disease they seal up the barn for days or weeks untill the corpses start to decay and then can be disinfected and sent to landfill.