How is backlink created between two internet sites?

I don't know where you got this idea, but it's entirely unnecessary. The only cases I can think of where you shouldn't link to something are pages that are clearly intended to be private (member only or customers only) but have somehow not been protected properly.

I do recall that in the early days of the public, commercial internet, say 15 years ago, there were a few attempts by some websites to prohibit other sites from linking to their inner pages. The idea was that they wanted visitors always to start at their home page, so they put in their TOS that linking was permitted only to their home page and they went after folks who didn't comply.

It didn't stick. I haven't heard about anything like this for quite some time. There's a universal presumption that if you publish it online, and if you don't protect it, then it's okay for anyone to link to it.

By the way, I don't think anyone's answered the original question. The HTML code for a link uses the anchor tag. For example, look at my sig. Where the words Executive I.T. Consulting link to my website, the code looks like this:
Steve