you must be friends with other ISPs, or you can buy a big line from one of the big ones, which is what the internet guides will tell you to do, but that to me doesn't seem very different from just buying home internet from an ISP.
Discussion
I guess the internet guides are more worried about telling you how can you get the infrastructure required to sell internet to others, which is being an ISP -- but it is philosophically different from being a "self-sovereign internet node" which seems to be what you want.
I think the answer is that there is no such thing as self-sovereignty on the internet, it's a set of relationships based on different interests, not very open, and very very hacky, and ultimately you depend very much on your peers, you can't ever be independent in any meaningful sense.