Both Aluminum and Glass are chopped up into tiny pieces, then melted into a "slag". The high temperature and chemistry allows both Aluminum and Glass to be 100% as effective as their originals. Its a true, infinite recycling process.
In contrast: Plastics and paper are "downcycled". High-quality printer paper gets turned into newspaper, then newspaper gets turned into cardboard. Each step, the recycled paper loses value due to contamination. In effect, paper and plastic cannot be "purified" like Aluminum or Glass.
IIRC, Type6 Plastic can only be turned into the foam for carpeting, for example. Some plastics, like HDPE or Type5 (Polypropylene) are highly-recyclable... but no plastic is as good as aluminum or glass. All plastics undergo some degree of "downcycling", where each recycling iteration loses value.