I need to be able the total numbers of downloads for my app divided by OS(android/ios), the API docs are bit confusing, I figure I would need something like a "get managed/filter", call but does this functionality exist within the Dynatrace api?
If we understood you correctly, you would like to GET the number of your App download from play store & app store using Dynatrace API which you will feed it to a data source with other third parties data in it.
To answer it there is no direct Dynatrace API to get this details. You need to create metric through which you can get the value in DT API as well as you can use it in DT dashboard for display the value.
Are you using DT RUM, Done Mobile app instrumentation, which technology you are using, code written in which language?
Now the tricky part. If you are doing this from serverside monitoring. You need to check with your application team/developer to get the class, methods details along with the parameter where it is mentioned as app download that too as per your requirement based on IOS/ Android.