Ok, I think I get it now, helped by this other TT post: https://ttlc.intuit.com/community/business-taxes/discussion/s-[product key removed]yee-with-new-401k...
Essentially, 401k contributions (whether it comes from employee paycheck or from the employer such as matching) are put under Retirement Contributions (line 17). Therefore, they are subtracted from Wages & Salaries (line 8 so they are not deducted twice, even though in my mind this was part of the employee wages regardless how much an employee diverted to a 401k. Roth employee contributions however remain under Wages & Salaries. The W-2s Box 1 should be used for Wages & Salaries and then any employer or employee 401k contributions be put under Retirement Contributions.
