It can be installed in any solid core door that can be drilled through for it to be fitted and tightened in place with a flat edge driver such as a screwdriver .... which covers virtually all steel doors on the market.
Clear glass (that means un-tinted) is almost invariably more transparent than plastic. That said, your question seems not to totally take the viewer's intended use into consideration. For close up only viewing, for which peephole viewers are principally designed and intended, the distinction, if any, between a glass and a plastic lens is apt to be minimal to non existent ... but can change with age, impacts, and weathering, for which reason glass could impart better visibility depending on price willing to be paid. Conversely, plastic is typically more rugged and breakage resistant than glass. Such viewers, apart from cosmetic features, are not generally of such expense as to make their fabrication material much of an issue, if at all.
This viewer sees 1 way - out only.
here are some: https://youtu.be/o9Fm8vxdg_E
Here's the exploded parts diagram I found following devcolabs instructions. Hope this helps.
5/8" is written on the package. Use a drill bit or hole saw. Pilot hole 1/8" or 9/64".
Just installed it yesterday. Lens do stick out, but you cannot see the door itself. Overall visibility is good, I like it.
Glass with metal housing.