This is my first year filing as a resident alien and I don't understand why TurboTax is adding $1400 equivalent to the third stimulus check to my tax return.

As a resident alien for tax year 2021, you are entitled to the third stimulus of $1,400. If you did not get it in 2021, you are eligible to receive it as the Recovery Rebate credit when you file your tax return.

Please read this IRS document for more information.