Is it required to max pre-tax 401{k} contributions before making any post-tax contributions when doing mega backdoor Roth?

My new employer offers mega backdoor Roth, and because I've never used it an an investment before, I have some questions on how it works in practice...

I know that the limit on pre-tax 401(k) contributions is $23,500 and the limit on pre-tax plus post-tax 401(k) contributions is $70,000. I also understand that you can convert any post-tax contributions into your Roth IRA, and this is the backdoor part of the strategy.

However, what I don't fully understand is whether or not I can contribute both pre and post-tax in parallel, or whether I need to do things sequentially and only start contributing post-tax after reaching $23,500 from pre-tax contributions?

Maybe it doesn't matter too much one way vs. the other, but the way I look at things, I could theoretically set up both pre and post-tax investments once at the beginning of the year, and not need to adjust everything mid way through when I switch from pre-tax to post-tax contributions.

Hopefully the question makes sense to people and people get what I'm trying to do here. Thanks!