Supplier
Supplier is whoever actually produces the engagement behind a service, using its own accounts, devices or software, as opposed to the panels that only resell and forward orders.
Every order ends somewhere. However many panels it passes through, one party at the end runs the accounts, the automation or the paid activity that makes the followers, likes or views appear. That party is the supplier. The panels in front of it set prices, names and terms, but delivery quality, speed, how the accounts look and how quickly counts drop are properties of the supplier's supply.
The buyer rarely sees the supplier. Panels treat their sources as commercial secrets, and a panel may not know whether its own provider is another reseller. Service names, quality labels and descriptions are rewritten at each step, so two services with different names on different panels can come from the same supplier, and two services with the same name can come from different ones.
This has direct consequences for your money. When a supplier runs short of capacity, every panel reselling it slows down at once, and orders sit pending or in progress whatever the panel's average time says. When a platform removes a supplier's accounts, the drops hit every order it delivered, on every panel. And a refill promise holds only as long as the supplier keeps refilling upstream, unless the panel you bought from is prepared to pay for replacement delivery itself.
What it does not mean
Supplier is not a synonym for the panel you paid, even when that panel calls itself a provider or says it owns its services. Owning a service in the sense of naming and pricing it is different from producing the engagement.