Is there a better way than this for configuring the defaults for the kapt plugin and the android extensions for all modules?

I usually prefer https://docs.gradle.org/current/javadoc/org/gradle/api/plugins/PluginContainer.html#withId-java.lang.String-org.gradle.api.Action- it's more abstracted than using classes, which might change between releases

How can I find out the id?

it's in the jar of the plugin, somewhere under

META-INF

alternatively, it's whatever id you use to apply the plugin

Ah nice, so it would be just

kotlin-android-extensions