Documentation lives with the project. Setup steps, environment settings, how to deploy, how to restore a backup, and where each credential is kept (never the credential itself in a document) stay with the code and are updated as the platform changes. A handover is far easier when that material already exists than when it has to be written in the final week.
Your product and the services it uses are separate. Owning your application doesn't make you the owner of the payment gateway, the email provider or the AI model it calls. Each has its own licence, pricing and terms, usually billed to you directly. We list those dependencies so the two never get confused when costs or contracts come up.