The IRS calculates what you would have gotten as your AGI if you had filed in 2018, from the information they have received from your various employers etc.
Upside, you will most likely get money from 2018, and possibly 2017. Trust me, if you owed the IRS money, you would have heard from them by now.
You should file 2018 now, and 2017 as well. Problem is, you have to file them by mail. And it will take a bit. But they will eventually get you your refunds, and the stimulus payment. You can wait for them to settle, and e-file 2019 at that point, or you can just go ahead and file 2019 by mail now as well.
And life lesson to share with your friends and family. Always file your taxes. Especially if you are on the lower end of the economic scale. The tax system is actually a great big pool of taxpayer's money. Not the Fed's money, the taxpayer's. It just sounds scary to do the work, and they don't make it easy. But there is tons of money that the IRS knows it owes people. And they sit on that money for 4 years and then, when you can't late file anymore, all that cash goes into the Federal coffers.
