can I make the banner stretch for different resolutions to make it look the same always.
and can I adjust the position finer than BannerPosition.BOTTOM_CENTER etc.
Currently it is not possible to change the banner dimensions. These are the images that are provided by the advertiser.
If you are using Unity Ads via the Unity Engine, then the only position options are via the BannerPosition enum and Banner.SetBannerPosition method.
If you are using Unity Ads via a native iOS or Android app, then you have more fine-grain control of where the ad can be positioned via the BannerView API:
https://unityads.unity3d.com/help/ios/integration-guide-ios#banner-ads
https://unityads.unity3d.com/help/android/integration-guide-android#banner-ads