Help with Admob mediation and withdrawal

Can someone please explain to me how AdMob mediation works?

Is AdMob taking any percentage from the income or is it absolutely free?

When I want to withdraw the money from that second ad network, do I get it from AdMob or from that network?

And is it better to use AdMob mediation or add second ad network directly in the app?

  1. Admob takes money for their impressions from advertiser not from your portion.
    2.second ad network pays you the money from second network
    3.mediation is good than second network though I use heyzap mediation in my apps.