When does employment begin? (employment at will)

As far as I know, after an offer letter is signed, the employer usually requires the completion of a background check, and gives the potential employee the W-4 and I-9 forms to complete, besides a bunch of other stuff as part of the onboarding process. Here are a few questions:

- After completing and submitting these forms, is the potential employee considered as hired?

- Can the company withdraw the offer at this point, or can the (potential) employee reneg the offer?

- Can both parties do these things without facing consequences (for employment at will cases)?

- When does employment officially begin, and when does someone become part of a company's employee records?

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