in the us they have all your info, but your employer pays an estimated amount of taxes out of your paycheck all year, and you’ve still gotta fill out paperwork about it yourself as well.
if it turns out you overpaid, which you only know by doing the paperwork yourself, and you filed taxes, you get money back from the gov. if you overpaid and don’t file, the gov just keeps your money. if you underpaid and filed, you’ll have to send them more money, and if you underpaid and didn’t file, the IRS will be coming for you.