Why do you use account with billing permissions to start any app?
As best practice, a consolidated billing strategy is best where all AWS accounts are paid through one master account. Only IAM users that need billing access should get access to it.
An app runs on an, e.g. GCE or a VM, that VM is a resource within a project. For GCP to deliver your bill for running resources, a billing account should be created and linked to your project, and then, you can create resources aka VM, buckets and others. OK?
Let us know. Cheers.
By starting an app if you mean creating a resource like deploying a new GCE VM or deploying an app to App Engine, then you shouldn’t need to have billing permissions on that project. Rather, what you will need is the necessary privilege to create that resource under that project.