Its about whether you can file as a US resident or nonresident for your taxes. If 2021 is the first year when you are present in the US, and if you only stay for about two months, you will be filing as a nonresident on your 2021 tax return next year. As a nonresident, you would need to file a US return only if you have any US source income and meet the filing requirements. For details, click here: https://www.irs.gov/forms-pubs/about-form-1040-nr
However, if you meet Substantial Presence Test SPT, you will be considered as a US resident for tax purposes. (Your last question) You can use TurboTax to file a Form 1040 and must report worldwide income including US and Mexico earnings.
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