I need help verifying my payroll is correct

I am an employee and my employer made several mistakes on several different paychecks I received this year. Part of this was due to the fact that I started the year as a non-exempt salaried employee, and was changed to hourly in July. Another complication was that after COVID hit, my hours were reduced to half of what I was working before, and I received unemployment compensation for a few months for the difference. This all resulted in me having to repay my employer almost $3,000, but, I never did understand quite how they messed up so badly, or if the amounts they came up with were accurate. I want to make certain that they were correct, or incorrect, and also make sure that all tax withholdings were corrected accordingly, so that I do not end up owing money when I file my 2020 tax returns.