How to deal with Android Manifests

What's the best way to deal with android manifest files?
I've had multiple projects in the past and I always notice the more plugins I introduce the messier it gets. End up with multiple manifest files everywhere.

Then your project is susceptible to the usual 'Unable to merge manifest files' or you end up with a duplicate Jar file somewhere.

At the moment I'm having the issue where we already have Google play services, Google Mobile Ads, but I'm trying to integrate Firebase. But I'm having issues with conflicts when building. So I guess my question is.

How do you guys manage multiple plugin integrations? Do you use 1 manifest file and just simply add to it?

Thanks