I've travelled on AZ from LIN (I think - or somewhere with hardly any airbridges) with my son who was in a wheelchair (broken leg). We were driven in a special van with some form of scissor-lift system that lifted us up to the aircraft and allowed him to be wheeled up to the aircraft door. (Of course being 5 years old he thought this was the way to travel and was rather disappointed to discover buses, queues etc in later years.)

Presumably something similar is done for wheelchair passengers when buses are used?