The stimulus payments received are an advance tax credit based on the 2020 federal tax return. You have to report the stimulus payments received because if you do not the program will give you the maximum tax credit you were eligible to receive, reporting that amount on the Form 1040 Line 30.
If you received stimulus payments and did not report that you received the payments and filed the tax return then you would be commiting tax fraud.
No they are not taxable and you don't have to pay them back. You enter them to see if you got the full amount you qualify for.
The Stimulus Checks are really based on your 2020 return we are filing in 2021. So any amount you already got will be reconciled on your 2020 return line 30. If you didn't get a Stimulus Check (and qualify) or you qualify for more it will be added to your 2020 return. But the good news is if they paid you too much they won't take it back.