There are many levels to explaining it but the simple way is yes, they do basically add money to the supply out of thin air when they do these stimulus packages, by both psychically printing money and adding to the tallies of total digital money, which in turn raises our national debt, which is borrowed from many places, like other countries for example.
This dilutes the supply of US dollar in circulation, which definitely will cause some amount of inflation in the market because the more dollars out there, the less scarce and the more goods and services will cost. Simple supply and demand. But economists argue there are ways to 'ease' inflation with subsidies and other financial trickery.
Some of it will work, at least temporarily. But there is no way around it, if you print trillions of new US dollars and add that to the current supply it will make the dollar weaker. Luckily its the world's currency standard and should remain so for a long time, so it's not insane to do. Basically we were forced to forego responsible economics because of a global economic catastrophe. Triage logic.