Vendors are generally small teams of people, sometimes single people that usually get products from manus to people in their area.
Depending on the vendor, it could be their own product like a board they made, a more general product like gateron switches that almost every vendor has, or other people's products.
In the case of other people's products, its usually because either the person can't handle logistics by themselves or it's because they want to sell worldwide. Vendors can help you get a product on their shelves although you'll have to contact them and sort everything out. Usually, it's a group buy item as vendors don't always want to buy your product without any prior interest, but you could just send it to them and split profits.
