What Product Flavor does Android Studio build by default in build.gradle?

Yes, there is a way:

android {
    productFlavors {
        foo {
            isDefault true
        }
    }
}

And otherwise variants with the debug build type are favoured.

It was added in Android Studio 3.5, see feature request:

"Included in Android Gradle Plugin 3.5.0-alpha08 and Android Studio 3.5 Canary 8 [3.5.0.7].

The heuristic for projects using older AGP and projects without explicit settings has also been updated to favour variants with the build type debug, as described in the commit message."

Currently there appears to be no way to select default flavor. There is a feature request in Android Issue Tracker for it:

https://code.google.com/p/android/issues/detail?id=64917

You can star/vote the request, so it gets higher priority on Android development tools backlog.

On the lower left of the Studio window there's a docked view called "Build Variants".

Open it and choose whichever variant you want to work on. This will impact which variant is built but also the enabled source folders, which will drive completion in code and resources editors.