"because last year dependents that couldn't claim the stimulus in the previous year, because they were dependents, filed their own return the next year to get the stimulus. Is that the case this year?"

Did you go to those links and follow along, or just fill in their forms to see what appears?

No one last year claimed a stimulus in the previous year. It's like this:

Either a 2019 or 2020 tax return is used to project what tax year 2021 filing would be for a person, to determine if they were going to be sent an advanced payment.

Then, their 2021 Actuals, on the tax form, is what you reconcile to, against what they got, if anything.

And for EIP #3, the qualifications have changed.

No one knows these people except for you. You have to look at the info or fill in the info.