Unity recommended to use Ads mediation, it will help to maximize the ad monetization of app publishers by increasing their display ad fill rates. But if you not going to use ads mediation you can setup admob separately
Installation
Download the latest Admob release
Run and import downloaded package
Setup
Go to your Google Mobile Ads account
Add new app to monetise.
Copy app id and ads id and past it into to Admob Android or iOS profile
Put your app id to Assets > Google Mobile Ads > Settings
