You are an independant contractor and file your taxes as such. Depending on how much you make, you may need to file quarterly. I use Turbo Tax for small businesses/ self-employed and find it tremendously helpfuol. (Plus the cost of it is a deductible business expense). (I am not a tax expert but just saying what works for me over the years).
No, Brent H., as a US-based freelancer it is my experience Upwork doesn't typically withhold anything related to government taxes, programs, etc. from distributions related to client payments for freelancers' work on their projects.
Upwork does reduce the amount paid to freelancers by the amount of fees due to Upwork, as outlined in the "Get paid on time" section of this page:
The same schedule applies to all project and payment types: hourly, fixed price, bonuses, etc.