to understand.

For simplicity's sake, we'll assume one job, no additional jobs or sources of income. Based on your W4, calculations are done to estimate how much money needs to be withheld for taxes. They'll know how many paychecks they'll send, they'll know what your total income will be, and therefore how much you will will owe for taxes. They withhold the appropriate amount based on your W4 information. Of course, having multiple jobs complicates this a bit, but the IRS withholding calculator tool can help you fill out the W4(s) for those scenarios so either way they know how much you're making and have a good idea at how much you should owe.

The thing is that bonuses are not "regular", even if they are, meaning that even if you reliably and consistently get a bonus, bonuses are usually by their nature not your normal income and are seen as one-off payments. Rather than try to throw that wrench into the calculations, they don't mess with how much tax they withhold from your regular paychecks and they just do a flat 22% withholding rate on bonuses to keep the math easier (so they're not saying "wait, we expected him to have earned this much by now, and would make that much by the end of the year, so now we'll have to re-adjust"). On a side note: YOU can do that by going to the IRS withholding tool, put in your bonus, and see if you can adjust your W4 to make up for it so taxes going forward should be the proper amount, but like I said that is something you'll have to calculate and change via W4, not something the company will do.

Come tax time, you'll figure out how much you were paid, how much taxes you should owe based on how much you were paid, and how much tax you have already paid. If there's a discrepancy in how much you owe and how much you've paid, you'll pay extra or receive a refund to make up the difference. So basically, the only difference a bonus would make come tax time is make it more likely that you'll have underpaid or overpaid, since they don't try to get the "right" amount of taxes out, they just take out a bunch and figure it will all work out come tax time.