Unfortunately taxes are not taken out automatically when you sell your stock for profit (I wish they were!). You will owe capital gain taxes quarterly if you make more than your typical tax refund would cover. You will need to run a cost basis report in your brokerage account to determine how much you made, and add in any dividends that may not be listed on it. I have heard from friends who use it that robinhood is difficult to determine your cost basis quarterly, but other platforms like fidelity and TD Ameritrade are pretty easy.
Here is how i calculate what I owe on a quarterly basis:
Let's assume your W-4 has no exceptions so your employer withholds the full amount of your taxes out of your paycheck. This means that a typical person is only working with the standard deduction as their buffer on quarterly earnings. If you are single that is $12,550 in 2021, and if married it is $ 25,100. So what I do is divide that number by 4, and that is my quarterly tax "buffer". So $12,550 / 4 = $3,137.50. $3,137.50 is the amount I can make in capital gains per quarter without worrying about about paying taxes. If i makes less than $3,137.50 in any quarter, I make no payment to the IRS.
But if I made $10,000 in capital gains in Q1, I subtract out the $3,137.50 to determine what I owe taxes on. So $10,000 - $3,137.50 = $6,862.50. In the 24% tax bracket, that would mean I owe the IRS $1,647. So you would go to IRS.gov and make an estimated tax payment for that amount. If you do not, you can get penalties at the end of the year when you do your taxes.