It's pretty straightforward for most people if you're filing for yourself. It only gets complicated if you do itemized deductions which only makes sense if your total deductions are greater than the "standard deduction" (which I think is like $12,500 or something? The tax app does it for you).

You decide how much withholding that your job will do when you are hired (you can have it changed if you want). You can choose to have them estimate based on how many dependents you have and deduct that from each paycheck. It will usually be slightly more than you actually owe, so you get a return when you file once a year. Or you can choose to not have them deduct anything, and then pay the IRS the total amount when you file.

The latter is probably the smarter way to do it in that you're not essentially loaning money to the IRS, but in a country where most people live paycheck to paycheck, people don't like the idea of having to write the government a 4 to 5-figure check every year.