Shouldn’t (not Should’nt) is the contraction of Should not

The location of the tag in the sentences doesn't change the meaning. In each case, the meaning is, "Don't you mean...?) * Don't put a space before a question mark. 1) 13 million? Isn't it 30? 2) Hardware? Should'nt that be software? 3) France? Don't you mean Belgium? 4) Bing? Isn't that Google?

Tag questions like "isn't it?" are used to confirm whether the preceding sentence is correct or not. If the phrase is positive, the tag is negative. If the phrase is negative, the tag is positive. The tag will use the same auxillary verb as the phrase. If there is no auxilliary verb in the phrase, use a form of "do". John likes you, doesn't he? That was a boring party, wasn't it? They are moving to another city next week, aren't they? He doesn't like when people interrupt him, does he? The teacher bought the statue in India, didn't she? You don't travel much, do you?

Well, How can I know when to use which one ? As I don't see any difference when I can interchange all the four of them ?

The examples are all correct so you don't need to worry. Here's more info on question tags: https://learnenglish.britishcouncil.org/intermediate-grammar/question-tags