don't isopods just eat leaf litter and rotting wood?
Ispods, at least the ones we use in the hobby are Detritivores, they eat Decaying matter, Plant/Wood/Animal. They also consume Animal Feces, and pretty much anything they can find. They will also eat molds and fungus somewhat.
They are "Decomposers" the trash recyclers of the ecosystem.
Springtails are microbivores mostly, but they do dabble in decaying and plant matter, making them all 3. Their preferred diet, is bacteria and mold, but they will vary for nutrition, and in absence.
So when they are fed rice (common food item) they don't eat the rice they eat the mold, and bacteria that grows on the rice, as it decays in the water, and becomes riddled with bacteria and molds.
The reason that people like to keep them in a charcoal substrate is the same. Charcoal is a natural filter. If added to your soil, it absorbs the nasties, such as bacteria and pathogens, molds ect. The spring tails, eat this bacteria and such from the charcoal, so the charcoal becomes a big ole buffet of pathogens. Its akin to keeping supers on bran, it provides there needs as a substrate, in that they can burrow and hide in it. However it also provides a food source.
You can supplement food for both, with similar. Their is CUC food on some sites like joshs frogs, that is a decaying matter ball, that then molds over and becomes even more food. Fruits and veggies work, as would lizard poop. If you have some extra, or even roach frass or other decaying herbivore poo.
Isopods, will enjoy some calcium source as well. Processed and crushed organic egg shell, whether reptile or chicken, as well as cuttlebone, or just some calcIum dust in their dirt, will be appreciated.
Isopods are not insects, they are Crustaceans like Crabs, Lobsters, Shrimp ect. There shells are Calcium heavy, and require calcium to grow and thicken.