What happens to mix up of post tax contribution to roll over traditional IRA?

I was contributing post tax money to roll over traditional IRA as I am above MAGI limit for ROTH IRA. I read about backdoor Roth IRA conversion but since I already contributed to existing roll over traditional IRA, what happens when I start withdrawal in retirement? I read about pro rata IRS rules on pre and post tax money withdrawal.

Can post tax money be selectively added to Backdoor Roth IRA conversion? Going forward looks like I need to open a traditional IRA and then do the backdoor Roth conversion.

Thanks in advance.