The IRS knows:

"If you were claimed as a dependent on someone else’s tax return for 2020, you were not eligible for the third Economic Impact Payment. If no one can claim you as a dependent for 2021 and you are otherwise eligible, you can claim the 2021 Recovery Rebate Credit, and must file a 2021 tax return to claim the credit."

https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-b-e...