On the tax amount, if you're interested in the details:

https://www.nta.go.jp/english/taxes/individual/12006.htm

I didn't go through all of it, but I'd expect it to be a tad less that 10% of the gross income.

If you're a US resident, I think you pay them as well on that same income ?

Not sure about the first part, but generally the way it works is that first you pay taxes to the country where you are a resident, and then you still must also pay taxes to the US by virtue of being a US citizen but you usually get a credit on those US taxes in the amount you already paid to your country of residence. Sometimes the country where you’re living has much higher taxes, so you end up not owing anything to the US — but you still have to file.