I do not understand why finding the minimum of elements take O(n) time
Sorting the array using comparison-based method takes $\Omega(n \log n)$. So sorting is obviously more expensive than finding minimum in $\mathcal O(n)$.

In fact finding minimum or sorting takes $\Omega(n)$ to at least read the whole array. For finding minimum it is actually $\Theta(n)$.

If it were true that sorting takes $\mathcal O(\log n)$ then you would be right to use sorting instead.