I think they usually use some variant of stevia extracts. It depends on the person, as different people are more sensitive to one thing or the other. Another big factor is what other ingredients were used, usually preservatives and stabilizers. People make big scares about some of these, but they have been used for many decades and are totally safe. But again, depending on the person, a specific combination can trigger more intense effects. Coconut oil is often used as well, which can be a trigger for a lot of people. By far the biggest factor is the dosage. As with anything bad for you, the dosage matters. A little packet of stevia powder in a cup of tea might very little, whilst candy usually has a lot of sugar and in turn a lot of sugar substitute. Eat just a few gummy bears and you’re fine, eat the whole bag and be prepared to live on the toilet for a while.
Another product often used, especially in those infamous gummy bears is Lycasin. For some reason lycasin is especially hard on the intestines, they want it out of there pronto and the best way to do that is to flood everything with water and violently eject it out of the nearest airlock.
Aspertame usually doesn’t trigger anything, but isn’t as sweet. So it’s mostly used in drinks, where other flavors and the carbonation is dominant in the taste. Drinking a couple of liters of diet coke containing aspertame is fine for most people, the acid and gas would be a larger source of discomfort than the sweetener. Eating a couple of tablespoon of the stuff would most likely trigger a response down below.