system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: We get Internet from ISP. From where does ISP get Internet and how?
gpt
Your ISP likely gets its internet connection from another ISP. That ISP may also get its internet connection from another ISP.  ISPs are generally considered  Tier 1, 2, or 3.

Your ISP is likely what is considered a Tier 3 ISP meaning they focus on providing the last mile to internet users. They then likely connect to another much larger provider who also connects other Tier 3 providers.  This provider is generally considered a Tier 2 and then connects to a Tier 1 which are your backbone Internet providers. 

Tier 1 providers don't actually pay for service. They have invested so much into their infrastructure and are so large that they enter into agreements with other Tier 1 providers to form the backbone of the internet. Tier 2 providers may blur the line between 1 and 2 and enter into similar agreements or may pay for service from a Tier 1 provider. 

Edit: 1 exception to this is Comcast which blurs the line between all 3. Also ATT, Sprint, and Verizon are considered Tier 1.