Although aluminium is bad conductor of heat,how can aluminium foil shining surface can be used to keep food hot do long time?
The answer to your question is actually hidden in your question itself.
Aluminium is a good conductor but as it’s shiny it’s a very poor radiator and absorber (these two go together). The inner surface, where not actually in contact, will not absorb radiated heat well nor the outer surface will radiate heat well. Hence the food will stay warm for longer time as it won’t loose heat much.