We received our second stimulus, into my account on January 4, 2021, does it apply to 2020 taxes or does it apply to 2021 taxes?

You report receiving the second stimulus on the 2020 federal tax return.

The stimulus questions are asked in the Federal Review section of the TurboTax program.  Any payment you were eligible to receive based on the 2020 Federal tax return but did not receive will be entered on the Federal tax return, Form 1040 Line 30 as a tax credit.  The credit is included in the federal tax refund or it will reduce the amount of federal taxes owed.

2020 taxes

Enter it on your 2020 return.

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.