What do employers do with the money they've withheld from you before they get sent in? Where do they deposit it?
The money gets sent to the IRS through the EFTPS system. Depending on the amounts, the employers are required to deposit it on a monthly/semi-weekly basis, so they don't get to keep the tax money for long.
Failure to deposit the payroll taxes on time is one of the most heavily penalized IRS offenses. This area of violations is called "trust fund violations". It is one of the very few areas in which employees may be liable for corporate misdoings (i.e.: an HR or accounting manager responsible for payroll may be personally liable for trust fund violations).