Regarding the multi-account diagram shown towards the end. The diagram makes it look like you can designate a separate account as your billing account. However, this is not correct. The "root" account in the diagram must be your master/payer account and it’s responsible for paying the bills in a single organization: https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/consolidated-billing.html
You are correct that the Master account in AWS Organizations is your Payer account and you can’t change this after you setup AWS Organizations. However, you can place that Master account in any OU you choose…even many layers down. Don’t believe me? Give it a try. Create some nested OUs and move the Master into those.
I always encourage students to lab something out if they have doubts before declaring that it’s wrong.