How do i fit the ads to the customer?

Hey, i've seen some games that ask for permission to fit the ads to the player.
I never thought that i need to ask the player first, because i never had the option to choose it.
My question is: How do i actually choose the ads in Unity service and in Admob service?

 

I think you can't. Its chosen by the server itself. Like in case of AdMob, Google use its AdSense tools to analyse and suggest best ads for every user. But then again I am not that much experienced at this. But i hope it helps.