May you please explain to me like I'm five these 3 things about adsense?

I'm new to the adsense world. I would really glad if you can help me to understand better the following things

1. What is an adserver? What an Adserver do?
2. How an header bidding software works? When you setup an header bidding server-side, how can the server-side software know which netowork is going to have the better earning?
3. I recently signed up to Google Adsense. But I get a warning about my ads.txt. So I did what they told me to do, but the warning doesn't go away. So I tried to take a look about the ads.txt of the site from which I'm taking inspiration from my website (this https://www.thequiz.com/ads.txt ) and I was amazed by the amount of services they are using. But, may you please help to me how to read better that file? For example why they have 3 different google adsense account associated with this website? And what is a reseller ads from google?

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Thank you very much!!!!

Submitted July 03, 2019 at 02:56PM by TommasoLi https://www.reddit.com/r/Adsense/comments/c8w5jt/may_you_please_explain_to_me_like_im_five_these_3/?utm_source=ifttt