If you can be claimed as a dependent---whether as a qualifying child or qualifying relative---- you are not eligible for a stimulus check. For the first and second checks, the parents of a child younger than 17 could get the "child" amounts of stimulus, or they can get it as a recovery rebate credit on a 2020 return.
The recovery rebate credit is only based on your 2020 return.
If you cannot be claimed as a dependent on a 2020 return you can get the stimulus money on that return. The IRS is not going backwards now and looking at 2018 or 2019----only 2020. And if you can be claimed on a 2020 return you will not receive the $1400 this year.