Odoo 19 Purchase has a built-in single-level approval based on amount. For several levels, you can add the Approvals app or Studio rules on top of it.

• Go to Purchase, Configuration, Settings and enable Purchase Order Approval, then enter the minimum amount that requires approval.
• Give the general manager the Purchase Administrator access right, because only administrators can approve orders above the limit.
• Keep engineers as Purchase Users so that, when they confirm an order above the limit, it moves to the To Approve state instead of being confirmed.
• For a second level, use the Approvals app with an approval category linked to purchases, or ask your partner to add a Studio approval rule on the confirm button with different approvers by amount.
• Test with orders below and above the threshold and check the chatter history.

This gives you control without slowing down small purchases, and every approval is recorded on the order.