Hi Terry,
Thank you for your questions.
In Start(), we start spawning our starting wave, which is a predefined wave (index). That is the idea the name startingWave is supposed to convey. Rick used two variables to make the code more readable in his opinion. It’s a matter of personal preference. Longer lines of code are usually harder to read than shorter ones.
In many cases, there are multiple ways to make something work in Unity, and he cannot show all of them.
You seem to have grasped the idea, and you seem to be able to express your idea in code. For this reason, I would say: Use your preferred version. That’s fine. 
See also:
- Forum User Guides : How to mark a topic as solved