How are nonresident aliens taxed?

Nonresident aliens are taxed only on the income that arises from sources within the United States.

Please be aware that the tax rate is progressive in the US and the bigger the earned amount is, the bigger the percentage you are taxed on is. You can check the IRS's official tax table here - https://www.irs.gov/pub/irs-pdf/i1040tt.pdf

For every state's taxation, the requirements and tax rate may differ so it is best to check the state's official website.