Odoo Purchase has a built-in approval step, and more complex flows can be added with the Approvals app or automation rules.

Options

• In Purchase settings, enable order approval and set the minimum amount that requires a manager's approval.
• Give approval rights only to the users in the purchase manager group.
• For multi-level approval or approvals by department, use the Approvals app with purchase requests.
• Use activities to notify approvers automatically when an order is waiting.

Keep the rules simple at first; too many approval levels slow down operations without adding control.