How Google detect any links that is relevant or not?I have heard that,Google is giving value only for the relevant links.Is it true? Is irrelevant linking is blackhat SEO technique?
Technically yes- a link created on a website with an irrelevant niche to your own, and done so for the sole reason of increasing your search engine rankings is a blackhat technique. WebDev
I think it's done words by words, if Google see that site is about dog and find some words that are not related I think that would trigger something.