Letter 6475 is telling you how much they sent you last year for the 3rd Stimulus payment. Not how much you are eligible for. If you are married each spouse should get a letter for half. So if you say no you got zero the IRS is still going to take it off line 30 and reduce your refund. They use your bank if you got a refund deposited on a prior return or they mailed a check or debit card. It might have been easy for you to miss a debit card and think it was junk mail.
Double check your bank statements and Check your IRS account (both accounts if married)
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
