I want to know the answer to the following question:
What is the simplest case of an ISP using an upstream ISP?
All the background I'm provided with is that "ISPs with more than one point of presence (PoP) may have separate connections to an upstream ISP at multiple PoPs, or they may be customers of multiple upstream ISPs and may have connections to each one of them at one or more point of presence.". Is that enough to answer the question?
Answer: no