Correct. If you filed as Married Filing Joint on 2023, to figure out your tax liability for that year, you will do a mock return for tax year 2023, filling as Married filing Separate (you won't be actually submitting this return). It means recreating your 2023 using your information only, and excluding your spouse's income to figure out how much would have been your individual tax liability.
Currently, you can only use TurboTax Online to file for the current year 2024. To prepare a prior year return using TurboTax you will need to purchase the download version.
File your 2023 taxes