There are metal recyclers all over the place that buy scrap metal. Look at any moderately-sized fabricator or tool and die shop for a couple of big dumpsters with the logo of your local recycler, they'll likely have one for ferrous scrap, one for aluminum, and probably a barrel for copper wire. If you have a big project such as replacing aluminum siding or 'scrapping' a trailer, definitely bring it to a metal recycler. Even if you're not a moderately-sized fabricator, it makes some sense to keep a box in the back of the shop for scrap metal, every year or so you can bring in a load of brake rotors, broken tools and auto parts, project cutoffs, electric motors, etc. and get a hundred dollars for your trouble while keeping those perfectly good metals out of the landfill. There's a level at which it transitions from a sensible ecological and financial policy towards an untenable sign of hoarding, undervaluing your time, and/or poverty but you can hopefully recognize when you cross that line.
Be aware that they'll be suspicious and will hopefully call the police if you show up with a few hundred dollars of nearly-new copper pipe or electrical wire that you cut out of an under-construction house, or anything else that looks like it might be stolen. Desperate people try that frequently, if you have the financial ability to post on HN it's not worth it.
In my city, the biggest name is https://padnos.com/, and https://www.grcentraliron.com/ has a nice consumer-facing business, but there are similar industries everywhere.
It's interesting to go down as a consumer and shop for cut-offs, you can get small workbench-sized sheet metal, 8020 in lengths not worth bothering with for a big fabricator but plenty for a DIY project, or bar stock to build something far sturdier than you can make with a 2x4 - all at prices per pound a fraction of what you'd pay at the big box stores.