Where will I be billed from when I auto-renew?

Your subscription will auto-renew from the original source of the payment. There are 3 ways to pay for the subscription (Website, Apple App Store and Google Play Store) and you're subscription will auto-renew from whichever provider you originally subscribed with. 

This means that if you originally subscribed via the Google Play Store but transferred your account to an Apple device for usage, you will still be billed from the Google Play Store going forward and you should manage you renewals from the original source.