If you're an employee of the UN, how does that work legally? Is your employer on the tax form written as the UN? Does it have a tax ID? Which country do you pay tax to? Is it treated like a corporation?
If you're an employee of the UN, and for some reason the UN decided that it's just not going to pay your salary, what can you do? As far as I know the UN has legal immunity, so... In that situation would you have no legal recourse?
Is your employer on the tax form written as the UN?
There is no tax form.
Which country do you pay tax to?
If you're a US national or permanent resident, you pay tax to the United States. You report the income as does a self-employed individual. Because the US is the only country in the world that taxes UN employees, the UN reimburses you for the tax so your compensation is comparable to that of its employees who don't have to pay tax to the US.
If you are not a US national or permanent resident, you are exempt from income and payroll taxation even if you work in the US.
Is it treated like a corporation?
No.
If you're an employee of the UN, and for some reason the UN decided that it's just not going to pay your salary, what can you do?
You can pursue your claim through the UN's internal justice system.
As far as I know the UN has legal immunity, so... In that situation would you have no legal recourse?
Beyond the UN's internal system, no, there is no recourse.
International organizations such as the UN are typically treated similar to foreign governments, and thus enjoy various privileges. One consequence is that typically, income from employment by these organizations is not subject to taxation by the host countries where the employee lives or carries out their work. However, the details depend very much on the tax laws in that host country. The employee might still be required to file taxes, but would then likely be able to declare the income as tax-free under the corresponding provision of the national tax law.
This usually also depends a lot on the treaties which created the international organisation. For example, the Convention on the Privileges and Immunities of the United Nations contains an explicit tax exemption for UN officials, though not necessarily for regular employees.
As an example for how a country treats employees of international organization, the IRS has guidance regarding US tax law here. Other countries tend to have similar rules, albeit with less accessible online documentation :)
