The stimulus checks are an advance on a credit you can receive on your 2020 tax return. If something went wrong or you did not get the stimulus check this year, you can get it when you file your 2020 return in early 2021—if you are eligible. It will end up on line 30 of your 2020 Form 1040.
The IRS will stop mailing checks on 1/15 so if you don't get it by the time the efiling opens at the end of the month you can apply for the missing money on the 2020 return.
CAUTION to those eager to jump the gun ... if you do NOT enter the correct amount of stimulus the IRS did pay you in advance then expect your eagerness to delay your return being processed. And since the IRS is still suffering from understaffing due to the covid situation and perpetual underfunding and they are also stretched thin managing the last minute tax law/form changes on top of sending out second stimulus payments expect the delays to be long.