Let's assume one company offers 200k salary and another 100k salary and 100k bonus at the end of the year. Is there a difference for me at the end of the year (after all taxes are returned)?
The reason behind the question, is that I have seen people telling that bonuses are taxed at the higher rate. Unfortunately upon a quick google resources claim drastically different numbers (from 18% to 40%), which confuses me.
Not in the U.S. (and I presume not in California, but I don't know for certain). There's no distinction on tax returns between salary income and bonus income.
What you're probably noticing is that income taxes are withheld at different rates for recurring income like salary and non-recurring income like bonuses.
Income tax on recurring income is withheld at a rate that would be applicable to that income extrapolated to the whole year. So if you make $1,000 in a week, that income es extrapolated out to a $52,000 income for a year and taxes are withheld at the effective rate for the year (after deductions and exemptions are considered).
Bonuses are additional income that is not included in the above calculation, so taxes should be withheld at the marginal rate for your income. Since your annual income cannot be inferred from bonuses, however, a static rate (e.g. 22% for federal tax) is withheld that is typical higher then the effective rate since no deductions or exemptions are applied (they're all accounted for in your recurring income).
Note that the calculation is not perfect, which is why you either get a refund or a tax bill when you file and all income and deductions are accounted for. So long as you didn't withhold too little, there's no penalty for being inaccurate in your withholdings.
This is a broad response for any country with a progressive tax system (basically all of them), which means that you pay incrementally higher tax for each new dollar of income earned.
In short, your average rate of tax on your income is lower than your marginal rate of tax. Your average tax rate = your final tax bill divided by your income. Your marginal tax rate is what an extra dollar of income would be taxed at. Your marginal rate is higher than your average rate, because your marginal tax rate goes up on additional income earned in a higher tax bracket.
Example: in California you pay ~2% taxes on income between 17k and 41k, and 4% on income up to 65k, and federally you pay 10% on up to $10 in income, and then 12% on income up to 40k, and then 22% on up to 80k. So from this information alone, if you had ~40k in income your rate would be about 2% CA + about 11% federal = ~13%. But every extra dollar you earn would be taxed 4% CA, and 22% federal = 26%. So that new dollar is taxed 26%, because that is your marginal tax rate. [Note - that extra dollar of income would have 26% whether it was earned by a bonus or regular wage].
This is even before considering your standard deductions, which further reduce your average tax rate, but have no impact on your marginal tax rate.
What this means is that when your taxes are withheld, they are withheld based on your expected salary for the year * your average tax rate. But when bonus taxes are withheld, they are taxed at your marginal tax rate at that point in time.
So if you look at your paystub with a bonus, it shows a higher rate of withholding, but that is an illusion. Your regular pay, if it increased that much for the rest of the year, would have the same ultimate impact on your net taxes, you just don't see it that way because you see your regular paystubs including the low rate of tax on lower chunks of income + standard deductions etc.
End of year bonus might actually be paid in the next year; this is common. So if you work all 12 months of one year, get your bonus, then quit in February of the next year, you have ~$100K income in each of 2 tax years.
