I worked for a company here in upstate ny that coated many optical and consumer items with aluminum...
Head lights , tail lights, stop , mirrors. Etc.
There are two methods ( maybe more), that coat glass, one method is flash aluminizing..
A simple explanation a part is put in a chamber pumped down to near zero vacuum. A Tunston element which looks like a large lightbulb element is loaded with a sliver of pure aluminum inside the loop. A high voltage / current is introduced to this element instantly vaporizing the aluminum depositing to the target. The chamber is normalized and part removed with gloves hands.... shiny... this is how headlights, plastic and glass are done.
The other is a bit more complicated, but higher quality. Cathartic Arc.. a clean part is put on jig or fixture placed inside a chamber under extreme vacuum, higher then space itself the parts are negatively charged inside, and a target metal is positively charged, the part passes in front of the target , positive metal is dissipated to the negative and the part is plated.
This process (simplified here ) is used from everything from titanium nitride coated drill bits and tooling, to gold plating jewerlery. And glass. Though not a conductor, can be done this way ,but may take many passes..
This process can bring the plating process down to a single molecule... there is more to this process, such as exotic gasses introduced. Various voltages, and frequencies.
Flash method is obviously cheaper , and usually does bulk items. The Cathartic Arc for extreme quality, precision, or metals other then aluminum or very high quality aluminum parts.
Both methods can be masked...
Flashlishlight reflectors use flash ( bulk and speed)
Headlights use either...
Tooling usually Cathartic arc.