Manufacturers roll the steel
A manufacturer operates roll-forming equipment, buys raw coil and produces finished panels. They can cut to any length, tell you exactly what gauge and coating you are getting, and they set their own lead times. What they generally do not do is install on your house. That is us: two lines, Farmville, NC.
Distributors stock and resell
A distributor buys finished panels from manufacturers and sells them on, usually in stock lengths. They serve a real purpose — local availability, mixed inventory, sometimes credit terms — but they add a margin layer and inherit their supplier's lead times. Many describe themselves as manufacturers on their website. Ask whether they roll their own; the answer is immediate and unambiguous.
Installers put it on your roof
Roofing contractors buy materials and install them. Good ones are worth their margin — installation quality determines whether your roof lasts twenty years or five. They typically buy from a distributor, occasionally direct from a manufacturer. If you want the shortest supply chain, buy materials direct and have your installer fit them, but check first that they will warrant labour on materials they did not supply.
Which one you actually need
If you are a contractor or building it yourself, you want a manufacturer. If you need a single sheet today and cannot wait, a local distributor may be quicker. If you want a finished roof on your house without touching it, you want an installer — and you should still ask them the manufacturer questions about gauge and cut length, because that is what determines what you end up with.