Simple answer: It's towards 2021 and you will benefit during 2022 when you do my tax return for 2021. It is highly unlikely to be part of your 2020 return, either original filing or amended.
For now, it's only a legislative proposal.. There is a provision for advanced monthly payments being made in 2021. The payments will come from the IRS, similar to the stimulus payments. The details of how and when are yet to be worked out, but starting in July has been mentioned.
