How are contributions to qualified plans taxes when withdrawn?

It depends on the type of plan and who contributed (employer, employee) and how they contributed (before-tax, after-tax).

For example, the rules for a Roth 401k are completely different than the rules for a traditional 401k.

In any case, if any part of the distribution (withdrawal) is taxable, it is taxed as ordinary income. But there are exceptions even there. For example, there are exceptions for Net Unrealized Appreciation (NUA) of employer stock distributed from a plan and there are separate rules for lump sum distributions taken by employees born prior to Jan 2, 1936.

The tax rules concerning retirement plans in the US are a horrible complicated patchwork maze. I don't understand how the average worker who does not devote his life to studying taxes is supposed to understand them.