What is inside these things, from a chemical perspective, and what are the kids breathing into their lungs?

In most cases insanely high concentration nicotine salts. Back before corpos got a hold of them, most everyone vaping used nicotine freebase in concentrations <20mg/ml. Freebase absorbs slowly, you had to work hard with 20mg/ml to get a headspin, like hyperventilating for a minute. Also less addictive. Salts absorb much faster, and the disposables (disgusting waste, but that's the profit model) are at 50+mg/ml, so the dopamine hit is a lot more instant. Of course, cigarette manufacturers have been finding additives to make their products more addictive since at least the 1950s, so whether or not and how long before those things make their way in is anyone's guess.

You can still get reusable vapes and freebase nicotine, and it's still the most effective way known for smoking cessation, but the enshittification has basically happened.