Hey there @Anonymous,
It looks like @john-pero already provided some awesome insight into how 401k deductions and contributions work. If you need to know how to enter this information into QBO, here is a useful link that explains the process step-by-step. Essentially, you can enter and make adjustments to 401k per employee from the Payroll module. This conversation thread might also be insightful if you need to make adjustments.
I hope this helps! And a warm welcome to the QB Community.