Is an underwriter used during pre-qualification or pre-approval of a mortgage loan?

I'm not sure if I'm in the process of getting pre-qualified or pre-approved. After taking all my documents and pulling my credit score, the loan officer said I qualify for the loan I wanted, but the underwriter would make the final decision. However, I did not get a pre-qualification or pre-approval letter. Does that mean I haven't been pre-qualified yet? How long until I get the pre-qual letter?

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