It’s all coming from H2O, but what is changing the molecules from liquid to vapor in each exchange?
Steam from boiling water is mostly pure water plus anything else that can evaporate. Dissolved minerals and salts stay behind in steam unlike mist from ultrasonic humidifiers. Those type vibrate the water so quickly tiny particles of water break off to form mist which is then blown by a fan which means any dissolved minerals will also break off with the water. That’s why these humidifiers tend to leave white dust behind and causes gas burner stoves to burn yellow due to the sodium content in the water.
Steam from boiling is vapor. It consists of single H2O molecules floating in air. Theres a lot and they start clumping together quickly to form the mist you see.
The mist from a humidifier is tiny drops of liquid water thrown into the air mechanically. each drop has millions of water molecules. If it’s tap water there’s also thousands of calcium and chlorine atoms. If it’s stale water ther may also be a bacterium in there.
There is no molecular difference at all, the only difference is in the temperature of the two. Steam is water that has been heated to over 100° C and cause to have a phase transition into water vapor. Your humidifier is encouraging evaporation, which is also a phase transition from a liquid into a vapor, however this phase transition happens below the boiling point of water.
There are several explanations for why water can evaporate below the boiling point, but I think the easiest way to think of it is to think of the water being dissolved by the air. The same way you don’t need to melt table salt to get it to dissolve into water, you don’t need to boil water to get it to dissolve into air