They are saying the first round of direct deposit is going out this weekend. I have read various reports as to how it will work, no two the same. Generally they say:
The IRS used either of a person's 2018 or 2019 1040s for the first round stimulus (the $1,200 one, CARES act). (Whichever 1040 they had, as far as address or direct deposit info was what they used to sent out the payments.)
For Stimulus #2 ($600) they used only 2019's 1040 and didn't use the 2018 1040 information even if that was all that was on file and what they used successfully for Stimulus #1. If your 2019 1040 was not processed by the time they sent out stimulus #2, you will have to request the credit for it on line 30 of the 1040 for 2020. There was an option, open until 21 Nov 2020 (?) where you could put your contact/bank information into the IRS website online if you didn't have to file or if your paperwork hadn't been processed yet (if you had any clue that was the case) and that entry should have gotten you a payment for stimulus dos. That's been shut down since then and so the line-30 request is pretty much the only option to try to get that second and/or first payment.
For stimulus #3, I have seen info that says they're using the same method as they did for stimulus #2 - your 2019 tax-year 1040 unless they have already processed your 2020 tax year 1040. If you did not file for 2019 and have filed for 2020 already and your 2020 return has not yet been processed, you may be able to file an amended return to collect the $1,400 per person (payment 3 of 3). Because you probably won't get one even if you should (except for SS recipients). If you get social security, but have not filed for 2019 or 2020 (or they've not been processed yet) the IRS knows how to reach you and should be sending at least $1,400 via the information the SSA has on you. At least, that's what I've seen go by in the press. God only knows.
This last stimulus payment (#3) allows you to claim $1,400 per dependent, and dependents include college-age students this year. But unless you have them listed as dependents on a processed 1040 (either 2019 or 2020) it looks like you'll have to do the amended return to claim the credit for them retroactively. Assuming the IRS doesn't do something proactive, such as re-opening the online portal so that people could get their information properly input into the IRS database for maybe a "do-over" processing run sometime in the future, but I'm not holding my breath on that.
I have also read that Biden has issued a memorandum to the IRS to make all possible efforts to find people due to receive a stimulus payment and see that they actually get one. I would hope that would involve looking at the 2018 returns as well as 2018 and 2020. I'm assuming not.