The ACA uses modified AGI to determine what the premiums should be for health insurance.

The IRS says:

For purposes of the PTC, modified AGI is the AGI on your tax return plus certain income that is not subject to tax (foreign earned income, tax-exempt interest, and the portion of social security benefits that is not taxable). Use Worksheet 1-1 and Worksheet 1-2, later, to determine your modified AGI.

TurboTax will use the information in your tax return to calculate your MAGI for the current tax year.