[simonjgreen]

This is not a universal answer and I think deserves some correction.

1. Majority of ISPs do not host any cache for Google content

2. Credible ISPs do not have bottlenecks at the transit or peering level

3. Netflix makes use of much more local caching but their model works very differently to Youtubes

4. The concept of "internet backbone" does not really translate to reality. Peering is significantly more mesh-like than that, and transit more diverse.

Source: I have owned multiple ISPs, and still do.