the banners should position themselves automatically.
when u create ur banner ad on admob, you have a few options, to what type of banner it will be and where it will show, being it interstitial or small top/bottom banner, once u generated the ad all u have to do is copy the id of the banner and load it.
the position automatically is taken from your admob ad banner place settings, is not a Construct feature cause the ads are a loaded from the plugin that is using a script to call your admob ad places which is very much like an iframe... so there is no need for u to manually position them on Construct.
this was how admob worked since it started and i don't think it will change.
the short answer is check your ad banner on the admob dashboard and make sure you load the right one.
make sure also, you disable the ad place setting that loads any type of ad.
there are responsive ads, interstitials or just small banner ads, if u leave it to size"custom" it will automatically fill in an ad that is available for you which is random or based on your tags u put on it for you app.
it also depends on the fill rate admob has, which is pretty much low, im not sure if there is still that setting that offers u filling rate and displays what ur filling rate is on the account, last time i checked back 5 years ago on my account i had a 85% filling rate, however coming years that number lowered more and more as competition in the business increased.
(knowing your filling rate matters very much for the type of banner u will be using to show ads, video ads have the most fill rates now a days because everyone wants to entice people better, where a static image doesn't really convey that message u want to send that good. to simplify this, you might have ur small ad banner working on ur device but if there are no ads to fill in your ad space u won't see any on ur app, same applies to videos, interstitials etc, but small banner ads while cheap, filling rate is really very low, back when video ads started small banner ads had best filling rates, and interstitials also, but then internet boomed and technology advanced, faster internet = better loading/buffering times = more filing rates for video places, so almost every advertiser changed from static small annoying ads to video or interstitial.)
Hope it answers your questions and solves your issues.
Hi there im not sure how long you've been using Admob but you cannot set its placement via the google admob dashboard. This has to be done within the app of where you want to place your ads. If you use android studio, you can choose to place the banner ad at either the top or bottom by editing the xml file. The construct 3 admob plugin should've added these as events to show the banner ad at either the top or bottom or auto adjustment.