The withholding  is just like W-2 withholding...that 20% may have been more than enough, or not enough based on your total tax situation at the end of the year.

As long as you've already entered your full 1099-R form in the software, and entered  the Federals taxes already withheld in box 4...then you are getting credit for having already paid that amount.   If you overpaid, then any excess might end up being used to offset any other taxes you owe...for (perhaps) income that had no taxes withheld.

 But if you end up in the 25% tax bracket, you'll be assessed some more.     And if any state taxes were withheld, down in boxes 14,15,16, then those withholding $$s will be credited to your state tax forms too...as long as you entered the amounts in the proper boxes in the software.

Early withdrawal??   then another 10% penalty is tacked on...unless the withdraw was for COVID reasons....but the forms to exempt that penalty tax are not ready yet.....the IRS hasn't even released them, so TTX can't include them ..yet.....either...and for that you may need to wait 2 or 3 weeks and come back later to finish up that part.

Hmmm, just making sure my figures are correct. So lets say I took $500,000 and paid $100,000 upfront tax. Is the income amount putting me in that higher tax bracket causing another $50,000 tax debt, plus the 10% penalty of another $50,000? Total of another $100,000 tax debt for 2020?

Roughly right....except that 500k is in 35% bracket...could end up being more than 50k added tax.  a lot depends on how much other income you have to include, BEFORE entering the 1099-R data.

and the 10% penalty...yeah, another 50k unless you qualify for a COVID exception.