On my W-4 in the space that asked about extra withholding from each paycheck and I wanted $70 to be withheld from each paycheck. How does that work when filing taxes?

How does extra withholding from each paycheck work when filing taxes? Is it lumped into the federal tax return or the state? Do I have to itemize and somehow claim that I had extra withholding? How do I find out that I'm actually getting the money back that I had withheld?