How do the different types of middlemen link a producer to a user within a marketing channel?

A retailer is an intermediary who purchases goods from manufacturers or other intermediaries and then sells them to customers. . When merchants are large enough to buy in huge numbers, producers sell straight to them. 
From the producer through the wholesaler to the retailer to the consumers
A wholesaler is a company that distributes items to other companies as a middleman.
Since many consumer items, particularly luxury goods, transit via wholesalers on their way to retailers, this channel is known as the conventional channel. A distributor is a company that distributes items to other companies as an intermediary. Distributors, industrial customers, and other distributors may be among these companies.
From the producer through the agent to the wholesaler to the retailer to the consumer
Agents serve as functional intermediaries who do not take title to items and are reimbursed by manufacturer commissions. Commonly, the products that agents deal with are low-cost, purchased by consumers things.
Producer to Buyer in an Organization
The company's own sales staff sells directly to employees and customers, or enterprise customers, via this direct channel. The manufacturer can give professional and fast services to consumers, such as shipping, equipment setup, and maintenance, thanks to the short channel.
Producer to Agent, and then to the Buyer of the Organization
Operational materials, auxiliary equipment, minor tools, and conventional components are distributed through this channel by producers.