Is the stimulus money being sent to individuals just a loan you pay back at tax time?

I know if you haven’t filed 2019 taxes the gov’t will use 2018. My question is, is the money just an advance, and you owe it back at the end of the year?

Also, if someone had no income in 2018 and did earn in 2019 and still is in 2020, and hasn’t filed 2019 taxes yet, will the government fail to send a check? Or, will the government use current withholdings being taken during 2020 from a paycheck as an indication to send the money?