Federal tax - how does it work?

[True Federalist (진정한 연방 주의자)]

You only have to pay more money before April 15 if you haven't paid enough taxes (through withholding) for the year or you're self-employed. Most people probably get a refund on or before April 15, and likely think that giving the government an interest-free loan of their money is a good thing. It's not.

A lot of people can't avoid that. Like people who get paid heavily based on commission or people that get very different sized checks throughout the year due to bouts of overtime.

Actually, it is avoidable.  If your wages are based on commission, and/or you reasonably expect to have those bouts of overtime, you can take fewer allowances and/or ask to have extra taken out of your paycheck.

Alternatively, if you don't expect to make much it is legally possible to avoid having anything taken out of your paycheck except for FICA taxes and also save yourself from having to file a tax return at all if you are a dependent and don't have ≥ $350 in investment income.  Fill out your W-4 as "exempt" but don't do it unless you expect to have less than $12,000 in earnings and that you'll be a dependent.  If you aren't a dependent you'll qualify for the EIC, but you can only get that if you file a return, so even if you don't make much you'll still want to file a return to get that.

[True Federalist (진정한 연방 주의자)]

You only have to pay more money before April 15 if you haven't paid enough taxes (through withholding) for the year or you're self-employed. Most people probably get a refund on or before April 15, and likely think that giving the government an interest-free loan of their money is a good thing. It's not.

A lot of people can't avoid that. Like people who get paid heavily based on commission or people that get very different sized checks throughout the year due to bouts of overtime.

Actually, it is avoidable.  If your wages are based on commission, and/or you reasonably expect to have those bouts of overtime, you can take fewer allowances and/or ask to have extra taken out of your paycheck.

...that would just result in more being taken out and thus a return, hence the "interest free loan" even moreso.

I've been told by employees of Verizon retail stores that they receive two and sometimes three checks a month, but commission is paid only once a month. And commission is often half of their pay. In other words the check including commission is usually about three times the other check. There's no way to avoid paying way higher taxes on that commission included check.

Then go the other way and have extra allowances so that less is taken out.  My point is that it is both possible and legal to adjust your withholdings so that you have the desired level taken out during the year.  Just don't overdo it the other way as if you have too little taken out during the year, you may get socked with a penalty.

[True Federalist (진정한 연방 주의자)]

It depends on how much you had taken out and how much you owe. That said, for 2017 you need to have made more than $6350 as a dependent to owe any Federal income tax. If you aren't a dependent, you should definitely file if you had any earned income. If it wasn't a lot, then you might not owe any tax, you'd qualify for the refundable Earned Income Credit, but you must file to get it.

Also if you got paid as a contractor (received a 1099-MISC instead of a W-2) you need to file.