For 2020 you can put $19,500 of your own funds into a 401(k). That can be split between Roth and traditional any way you want. You will specify that split when you specify the contribution level through your employer. If you turn 50 this year you can add another $6,500, and that can also be split any way you want.
The 401(k) trustee will know the source of each $ in the account. They will know what portion is traditional, what part is Roth, and what part is company match; they even track the growth due to pre-tax and post-tax funds.
You can move this money around within the account. You can decide how to split the money among the fund options, you just can't change its source. They have to do this so when you make a withdraw or a rollover they can tell you the tax impacts.
when I look at the quarterly statement, It reminds me of my Vanguard account. In my Vanguard statement I have a section for Roth IRA, Traditional IRA and taxable account. For my current employer there is a section for Roth 401(k) and a section for traditional 401(k). Each section can split their investments differently.