It’s nice to see another person like you, posting here, that seems to understand the fundamentals of how a search engine actually works. There are a few on this board and I think it helps this community a great deal to have you and others jump in and point out what everyone should know if they have a grasp of the fundamentals. I agree with you that a backlink can be from any web document, including pages on your own website.
The most fundamental concept of SEO is understanding that search engines rank individual pages, not websites. The misguided notion of a “website” ranking is typically what leads to all kinds of hogwash that espoused on this forum. I would go as far as to say that “pages” not “websites” are what gains authority, and it is backlinks from authority “pages” that help boost our SERP rankings.