As to the original topic about backlinks...
Google is constantly improving on how they understand relevancy, and they use tf*IDF models more efficiently than pretty much anyone else on the planet to this end. To put it another way, they can figure out the level of relevancy better than ever before down to the level of a single word in a single sentence. From there, you can scale that upward:
- Single word
- Sentence
- Paragraph
- Section of a page
- Page as a section of a site
- The section of a site
- The site itself
- The narrow industry
- The larger vertical
Now that's the kind of thing that sounds all great and technical and can make you look smart if you repeat it. But here's the thing that actually matters in terms of what you'll see in the real world:
They get better at it all the time.
What this means is that you can't think about what's the best option for backlinks now. You have to think about the best option for backlinks one, two, five years from now when Google understands all of this better than ever before and tightens down even more.
In short, you have to make Google's updates work for your backlinking strategy instead of against it.