Confused regarding my tax filing status

[namage]

I entered the US in early Aug 2019 on F1 visa and have been here till date. on Oct 1st, 2021 my status changed to H1B visa. For current tax filing (for the year 2021) I thought I had to file  as non resident and resident as per the day I changed to H1B visa. But I came across articles saying I should only file as non resident for entire of 2021.
Is this correct? For both federal and state tax (CA) do I just file as a non resident for 2021? 

[namage]

Also, if non resident, does the tax treaty apply towards income tax?

[LoganathanB]

You must file a dual-status tax return for 2021.

You are a nonresident alien who’s changed to a resident alien and are a U.S. resident on the last day of the tax year.

 Form 1040 is your main return. Include Form 1040-NR as an “informational statement” and be sure to write Dual-Status Return across the top of it. 

Yes Tax treat will apply if you are a nonresident subject to what is in the tax treaty between USA and your country.

Click on the link for more information

Dual Status Return