Explanation
To automate applying discounts on opportunity products when an opportunity is won, the administrator should use Flow Builder, which is a tool that allows creating complex business processes with clicks. The administrator can create an autolaunched flow that runs when an opportunity is updated, checks if its stage is
"Closed Won", and updates its related opportunity products with discounts from a formula or variable.
Approval Process, Prebuilt Macro, and Formula Field are not able to update related records based on criteria.
References: https://help.salesforce.com/s/articleView?id=sf.flow_build_overview.htm&type=5