Joining arrays

Hi, I'm using Unity but I don't think it's important.
I have a list of object, on which I want to call a method which returns a regular array (struct[]) which can be quite big.
Then I want to join all those arrays into a singular array. That big array will be generated only once, not modified and used for grabbing a random entry from that array multiple times.
As I'm not that experienced in optimizing c#:
  1. Is there any advantage in using array here instead of List<T> which I feel is easier to work with?
  2. What's the best way to join those arrays into singular one?
So far I'm doing it like that, but I think it is not optimal as I create new array for each iteration?
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