Hi, and welcome to the site.
I'm afraid that it's a little unclear what you're asking about, as LED flashlights don't have bulbs. Are you asking about the construction of the flashlight body itself? That can sometimes be referred to as the housing. The part of the body that holds the lens is referred to as the bezel or head.
Most high-end flashlights are made out of aluminum, which is lightweight, easy to machine, and transfers heat well. (Heat management is important with powerful LED emitters.) Many aluminum lights have stainless steel bezels. There are also lights made entirely out of steel, titanium, copper, etc. - each have their advantages and disadvantages, and are usually more expensive.
There are a few advantages to plastic, but not many. It's inexpensive. It's bad at shedding heat. Plastic flashlights can be cheaper, but they're typically more fragile. Some specific industrial settings require plastic housings to reduce the possibility of sparks. That's about it.
I hope that helps. If you're just starting out and looking for a light, you may want to post in the "recommend me a light" section of the forum.