Why commercial gummies do not melt? I've noticed that commercial gummies do not melt, even when they are left in the sun, while home made gummies melt even at room temperatures sometimes.

What is the secret ingredient? Sugar?
I've been making a lot of room temp gelatins lately; not gummybears, something else with a bit lower gelatin concentration. The secret ingredient seems to be time. The first day, things are pretty jiggly, but even when hermetically sealed, things get tougher and tougher over a course of about three to four days. I expect it's some colloid maturation process involving random chains finding their best fit over time. Don't quote me on that, but the same sort of thing commonly happens on a faster timescale with gluten  maturation in bread dough.
A few hours in the fridge can speed up firming, but I'm not sure you end up with the same final gel structure that way.