It's usually around 40%
Don't confuse withholding with taxation. They may be either at 40% by the company, but are only taxed at the end of the year at your marginal rate.
22% fed + state. It’s standard tax so if your marginal is much higher you need to overwithold rest of year to make up for a larger bonus. The actual tax is marginal when filing taxes, but the actual bonus is what i said
Bonuses are taxed at a regular income tax rate, same as your paycheck. Withholding is calculated differently as pointed out above, but the actual tax is determined when you file the tax return (in W2 salary, RSU and bonus will all be lumped together).
I think the rate is much higher for non-resident aliens (e.g. student visa), regular tax rate otherwise. Not 100% sure though
Geez, no. https://www.irs.gov/individuals/international-taxpayers/substantial-presence-test
No about which part?
Your marginal tax rate. Practically all bonuses are taxed as regular income