Notice 703 is explained above. It is simply the calculation to determine if any of your SS is taxable. See the actual notice at the link below.
The IRS (nor TurboTax) is NOT making your entire Social Security benefit taxable. TurboTax will prepare an IRS worksheet to show you the calculation of how much is taxable. It is never more than 85% of your SS benefit.
Notice 703: https://www.irs.gov/pub/irs-pdf/n703.pdf
IRS worksheet for taxable SS benefits https://www.thetaxbook.com/updates/TheTaxBook/Client%20Tax%20Tools/2020_Social_Security_Benefits_Wor...
