Deductions & credits

I still don't get it clearly.

 

Let me put it this way:

1. 401k withdrawal amount: $15,000 ($2500x6, June-Dec. 2024)

2. Federal/state taxes due: $638 per each $2500 withdrawal, so the total is $3,828

 

If I choose to pay the taxes now, I would receive $1,862 each month after the tax.

If I choose to withhold the taxes until next year, I would receive $2,500 each month before tax.

 

Okay, let's say my taxable income for next year is $30,000, other than this 401k.  So, my total taxable income is $45,000($30,000 plus $15,000), right? 

 

Now, this is my question. During the tax return next year, should I pay the taxes on my taxable income of $45,000? Or should I pay $3,828 (401k withheld tax), in addition to the taxes on $45,000?  I'm confused here.