As an example I would like to look at how the builtin “PO” is configured in ERPNext. I see no items in the Workflow List? If this PO process is implemented in the back-end; why not use the Workflow tool provided?
There is no default PO workflow set in ERPNext. You need to create one using the Workflow tool:
https://erpnext.com/docs/user/manual/en/setting-up/workflows
Just to be clear; the PO lists (etc) are available for use, but no processes are yet implemented. But submitting a PO does (at least) change the status; so where are these mechanisms implemented?
It is a standard framework behaviour.
All doctypes which have Save + Submit action, have Is Submittable checked. This is a framework level behaviour.
The submit permission can then be given to the user via Role Permission Manager.