What is the best practice - To have Service Account as a Primary Owner of a Power Automate workflows or as a Co-owner or not have a SVC at all? What is the usual practice in some other organisations?

We use Service Accounts that have a non-expiring password for Power Apps.

Make sure that you keep a record of all Service Accounts in an online vault / locker.

Implement role-based access controls (RBAC) where roles assume the necessary privileges each time an automation is performed. This minimizes security risks by limiting privileged access to only when it's needed, ensuring a more secure and compliant automation process.

Best to avoid service accounts and permanent users for automation tasks.  These typically requires elevated privileges and are common to fall out of security compliance rules. Best practice is now to implement roles to assume the privileges each time the automation is performed.