The employer contributions go to your traditional account in the 401(k), not to the Roth account.
You can end up with all of the money in the Roth account only if the 401(k) plan allows In-plan Roth Rollovers (or if the employer makes no contributions). An IRR is a taxable rollover from the traditional account to the designated Roth account in the same 401(k). An IRR is reported on a Form 1099-R with code G in box 7 and the taxable amount in box 2a.
The 401(k) plan administrator must keep track of everything so that it can determine the taxable amount of any distributions.
