Congress passed the 2nd stimulus bill at the end of 2020. Both stimulus checks were an advance credit for your 2020 tax return, The 2nd stimulus checks started going out at the very end of December 2020 and continued into early January 2021. When you fill out the recovery rebate credit it is important for you to enter the entire total of the amount you have received for yourself, your spouse, and your dependent children so far. Entering the wrong amount might delay your 2020 refund.
It should help you if you bear in mind that the purpose of the question is to determine if you have already received the amounts you should have received.
12 February 2021
Thank you XMASBABY0 for your reply to my EIP2 question. I appreciate the time and effort you put into your reply. Unlike all other tax data that is required to be entered by the end of the calendar/tax year, now entering EIP2 amounts is a moving target. Other that the details shown in the TurboTax Recovery Rebate Credit Worksheet, I cannot find an IRS bulletin or notice that provides filing details. Questions left open for me are: (1) if an EIP 2 Rebate payment is received after one submits their 2020 tax return is a Form 1040X submission now required to show the payment? or (2) will the payment be entered in one’s 2021 tax return?, and (3) if one submits a Form 3911 (Taxpayer Statement Regarding Refund) for IRS reissue of a lost payment and later in the 2021 tax year will my questions (1) or (2) apply? Thank you for any further help you care to provide on this issue. Sincerely, KCM
To reply to your questions:
 It is unclear at this time whether they will automatically adjust the stimulus for returns that have reported it incorrectly. Therefore, yes, it would appear Form1040-X would be required for stimulus payment adjustments.
 No; The first and second stimulus payment reconciliation is done on the 2020 Tax return due April 15, 2021.
 Even though Form 3911 is submitted in 2021, it will not impact 2021 because Federal Income Tax Refunds are not taxable. State income tax refunds may be taxable depending on your circumstances.
The second stimulus remains tied to 2020 because the legislation was passed in 2020 as @xmasbaby0 stated. Tax law sometimes changes and is applied retroactively depending on what is written for the law. You could also think of it as an exception made for the second stimulus payment to be received in 2021 but apply to 2020 so that people needing assistance do no have to wait even longer to get the funds that can help them now.
These payments are not taxable. However, some states do count them toward household resources in calculating things like property tax credits.