This link is good to read
https://www.flightglobal.com/FlightP...20-%202323.PDF
a) A polishing product such as Wadpol, Wadpol is a tin of fibrous cloth pre-impregnated with polish is used in a rotary motion to the 'bright' areas such as leading edges and intakes. It is then buffed using a rotary buffer such as those used for cars.
b) different products can maintain the shine, but mostly after the finish dulls, re-buff or re-polish is required periodically.
c) the application of polish should not remove too much material so that corrosion preventative coatings are removed i.e alclad
d) Although some operators still have certain aircraft in bare aluminium it is expensive to upkeep looking good, American Airlines have a few in their fleet in retro livery.