Admob for Unity: clarification on use needed

It's actually a mess.

Unity Ads and Google Ads are two different systems and it's still recommended (from my and other users experience) to use Google Admob, because it simply gives you more money.

In order to have working ads, you have to:

1. Add them through the code
2. Add the app to Google Play without testing the ads (app may be in alpha-beta phase, don't worry, it won't be public)
3. Add the app to the AdMob console (https://apps.admob.com/v2/home)
4. Create ads in the AdMob console (video, static, banners) and get their keys.
5. Put ads keys into the code (see below)

When you have ad ID's, you put them here:

using GoogleMobileAds.Api;
…
private void RequestBanner()
{
    #if UNITY_EDITOR
        string adUnitId = "unused";
    #elif UNITY_ANDROID
        string adUnitId = "INSERT_ANDROID_BANNER_AD_UNIT_ID_HERE";
    #elif UNITY_IPHONE
        string adUnitId = "INSERT_IOS_BANNER_AD_UNIT_ID_HERE";
    #else
        string adUnitId = "unexpected_platform";
    #endif
}

as mentioned on the website you provided.

Then, you can test your app, even by building in Unity and uploading directly to the phone/tablet. Remember to put ad's ID's to the code. You'll have your ads working.