I ended up creating a massive comment describing physical connectivity between service providers, and figured you'd appreciate it!

Comment is here for you to check out!

OP's post was essentially "Why do I need to call intermediate service providers to troubleshoot BGP sessions going down?", so my reply is focused on that question, but I have a feeling it might bring up a lot of other questions about internet connectivity that others in this subreddit can answer. I feel like there's a massive gap knowledge gap between "how the internet works" from a routing vs. physical/business/everything-else perspective, and I'd like to shed a bit more light on that if possible.