Same way it happens outside china in other industries. You know a supplier who sells material / service for $BUDGET minus X, in exchange for swapping the PO you get a portion of X from the supplier via back channel means, while company is now paying $BUDGET for lower quality material / service that costs supplier less.
And as the other posters tell you, possibly failing QA checks is not stopping profiteers from attempting these kind of changes. Constantly. It's a numbers game. Expected profit based on how long until your swap is discovered.
If factory owners really want integrity from software-executed QA checks, they can require HQ authorization of software and config updates on factory floor equipment that is equipped for local tamper detection of control planes.