Using the definite article vs no article

I come across many sentences that I fail to undertand why the authors have used the definite article, the.

For example, look at the following sentence. This is the first sentence (in introduction) of a journal article.

The activities in a supply chain have the role of transforming raw materials into finished products that are
delivered from the supplier to the end customer.

I don't understand why the author has used the before supplier and end customer. I don't know who is the supplier or who is the end customer.

How about if we write in the following way. In fact, I have also seen such variants in the introductory paragraph of some artciles. Are the 1, 2, and 3 correct and carry the same meaning in this context?

1. The activities in a supply chain have the role of transforming raw materials into finished products that are
   
   delivered from a supplier to an end customer.




2. The activities in a supply chain have the role of transforming raw materials into finished products that are
   delivered from suppliers to end customers.




3. The activities in a supply chain have the role of transforming raw materials into finished products that are
   delivered from supplier to end customer.

I believe there are subtle differences that nonnative speakers can't understand. Could you reveal the mystery of the?