Based on the information you provided, you began counting "days of presence" in 2019, and if you were present for 183 days you are a resident alien for U.S. income tax purposes in 2019. Your resident alien status starts on the first day you were present in the U.S. in 2019. Social Security and Medicare Taxes start on the first day you were a resident alien, i.e., the first day you were present in the U.S.
See the section of IRS Publication 519 at this link for this guidance on your residency starting date:
Residency starting date under substantial presence test.
If you meet the substantial presence test for a calendar year, your residency starting date generally is the first day you are present in the United States during that calendar year. (emphasis added)
See the section of IRS Publication 519 at this link for the guidance on Social Security and Medicare Taxes:
Social security and Medicare taxes will be withheld from your pay for these services if you are considered a resident alien as discussed in chapter 1, even though your nonimmigrant classification ("F," "J," "M," or "Q") remains the same. (emphasis added)
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
