There are so many websites with millions of backlinks but few referring domains. Therefore, I wonder how it's possible to have millions of backlinks. Later, I found out they had over one million backlinks coming from one website alone. One million backlinks are impossible to create manually. What is the benefit of this and how does it affect SEO?
I don't actually have any evidence to back it up, but I do not believe there is any SEO value in receiving more than a few links from one site. And sometimes such situations exist because both sites have the same owner, and they are offering products back and forth, not necessarily going for link value. In this case though, Google does negatively impact the ranking. The two sites will be treated as one.
Yes, you said it right. If someone tries also it's impossible to get thousands of backlinks from one website. But I also don't know how it's possible.
It's possible. For Example, you are a web designing company and design an eCommerce or marketplace for large businesses and link back your website in Footer. So, every time a new product page or the new page will be added to the website it will give you a new link but these links are of no use (because of nofollow aributes and depends on lots of other factors). If you have millions of dofollow links from one website and there is no variation or other referring domain then those links have very low value or sometimes increase your website's spamming score.
The number of backlinks needed goes up at a faster rate than the difficulty ranking. For example, if a keyword has a difficulty of 10 you probably need about 10 backlinks to rank. But if it has a difficulty of 50 you may need 100 backlinks to rank.