Steam doesn't help with taxes, you are responsable for all taxes on steam, this is the most common thing when developers think they are getting the right infomation but are kinda messing up.

So am just going to assume you have a LLC or something close to a business recognize by the goverment France. You have a EIN so you are somewhere in owning a business with real value. This is not to confuse with sole trader where your stuff is not protected.

So you won't pay VAT unless you reach 35,000€ ( https://mr-eurodisco.com/finance/french-vat-rates/)

Must pay 33% corp tax unless you are brand new or unable to pay 3,000€ https://www.expatica.com/fr/finance/taxes/corporate-taxes-in-france-and-french-c...

Withholding tax is determinte by France tax authorities there for i do not know (https://www.welcometofrance.com/en/news/practical-details-of-the-withholding-tax...)

Income tax brackets in France for 2019 are as follows:

Up to €9,964: 0%
€9,964–€27,519: 14%
€27,519–€73,779: 30%
€73,779–€156,244: 41%
€156,244+: 45%
You can deduct certain things before calculating tax, which include a 10% deduction for work-related expenses, social security contributions, interest on certain business loans, and French pension contributions.

IRS if over $400 or if company meets requirements (https://www.irs.gov/businesses/small-businesses-self-employed/business-taxes)

Those are the tax for LLC for france has far as i know. As well has IRS. You only have to worry about IRS since the threshold is low and Withholding. This is all based on me thinking you own a company with some Value, not an answer to why you might get tax.

It is possable you won't get tax with "Pay by itch" but that doesn't change anything if using "collect by itch". Both are the same, the only diffance is how it was handle.