This is more like a topic you should Google. The folks in this forum/community may know something about it, but you can easily learn about these concepts from sources on the web
I searched for: learning about ad mediation
and found a interesting article that sums ‘Ad Mediation’ up well:
https://venturebeat.com/2015/05/06/the-what-why-and-how-of-mobile-ad-mediation/
what is the difference between an Ad-Mediation Provider(Appodeal) vs. Your average AD Provider(AdColony, Admob, Appnext, etc.)
They are competitors and each have their own sources of ads an approaches to mediating. There is no simple way to describe their differences and (I think to answer your underlying question) which one you should use.
My suggestion is, forget about monetizing for now. Learn and make games for free. Then, when you feel you’re able to create something worth monetizing, use Appodeal.
Why?
As you say, Appodeal now owns Corona so why not help them stay in business. i.e. You’re using Corona for free, throw some of your earnings into keeping it viable by feeding the parent company.