The IRS letter should say what they changed. Sounds like they show you had more income than you reported. That is a lot. I would call the IRS and find out more. Compare your 2020 return with 2019, maybe you'll spot something you left out or entered in the wrong spot.
Turbo Tax is a self prepared return. It has no way of knowing or checking that you entered something wrong or in the wrong place or entered something twice Or left something out. It doesn't check against any outside sources. The IRS has copies of all the W2s and 1099s your employer and banks/brokers, etc. sent in to match against. Turbo Tax only checks the calculations of what you entered. It is your responsibility to check it over before you file. They only Guarantee that the calculations are right based on the entries you made.
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