How does adding big O notations works can someone please explain how adding big O works.
i.e. $O(n^3)+O(n) = O(n^3)$
why does the answer turn out this way? is it because  $O(n^3)$ dominates the whole expression thus the answer is still $ O(n^3)$

Equality 
$O(n^3)+O(n)=O(n^3)$ we can understand as equality between sets - they are two forms of recording the one and same set. Formally, left side is subset of right side and reverse:
$O(n^3)+O(n) \subset O(n^3)$  and  $O(n^3) \subset O(n^3)+O(n)  $.
If "dominance" is understood as inequality $n \leqslant n^3$, then we can say that, yes, $n^3$ "dominates" $n$.
