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.
This isn’t as protective as one would imagine. Unscrupulous floor managers will still swap materials, run the crap and sell the good stuff.