When you enter your 1099-B form, Box 4 contains the 20% Federal Income Tax Withheld. You'll get credit for paying it on your 2020 income tax return. The withholding is treated just like the withholding on your W-2. It may increase your refund or decrease the amount due.
When you sell a capital asset, the difference between its cost basis and the selling price results in a capital gain or loss. You have to pay income taxes on the sale of your stock, if you had a gain.