These links are purchased from youngsters on some hacker websites, they're very cheap. Like the article I submitted to admin5 last year, it mentioned a line: "Baidu reacts quickly but does SEO very slowly." If you want to do something, use a technique for it; if you use this technique to do something else, it might be more profitable. Of course, that article didn't have this sentence, it's just my summary. Since that article also discussed the concept of black-hat SEO, I brought it up. This article is just an introduction to black-hat SEO, so it doesn't go into much detail about SEO. The following few articles will focus only on the technical aspects and won't cover these theories.