This video is a Rare one showing a 777200ER with a INOP APU so the plane needs an air starter to start the engine to then get the plane able to be self sufficient and running. a normal plane would use one airstarter with one hose leading to the belly where it can be used for air conditioning
and for starting. this 777 had 2 airstarters and 3 hoses crazy

but today it was a Rolls Royce TRENT 800 being started