How to simplify Big $O$ notation?

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Here are my questions:

Simplify the following $O$-notation statements as much as possible, e.g. $O(n + 25 + \log(2n)) = O(n)$.

(i) $O(n^2 + n)$

(ii) $O(\log(2n)) + O(n) + O(n^2)$

(iii) $O(n) * O(\log(2n)) $

(iv) $O(2^n) * O(n^2 + 2^n + \log(2n))$

I know the first three answers are:

(i) $O(n^2)$

(ii) $O(n^2)$

(iii) $O(n\log(2n))$

However, I'm unsure on the last one. Would it be $O(2^n)$ or would it be whatever $O(2^n) * O(2^N)$ is, which I'm not even sure what that is? How do you simplify Big $O$ notations with multiplication involved.

Many thanks if anyone can help.