First of all, it sets up reporting via AdSense Management API: https://developers.google.com/adsense/management/getting_started. It works through OAuth2, and uses client_id, client_secret, refresh_token and first access_token.
Secondly, it creates ad units in Admob account and Appodeal server.
This extension basically imitates human work, so during the sync on the screen you can see what plugin does, what pages are opened, etc. Also, here you can check the extension code, it’s freely accessible: https://github.com/appodeal/admob-configurator
We strongly suggest using this extension to sync Appodeal with an Admob account. It automates everything on Admob and Appodeal side, and it is much less likely for problems to arise.