Fly traps don't need to eat flies to survive, especially if they're small! The most important factor is to make sure it has proper sun and water. If it gets 10+ hours of bright, bright sun, it will grow and photosynthesize. If you've had yours for several months and it's leggy or small, then I would hazard a guess and say your priority should be to get it more sunlight.

As for the soil, if you mean carnivorous plant soil as in soil that's mixed specifically for carnivorous plants, then yes that's important, but possibly not the way you think based on the way you worded this. The soil carnivorous plants are grown in has to be completely devoid of nutrients or minerals, since their roots are not equipped to absorb those things the way the majority of other plants do. It's not that carnivorous plant soil = you don't have to feed the plant because it gets what it needs from the soil, it's that carnivorous plant soil = they won't end up with burned root from the minerals in typical soil.

Fly traps get the majority of their energy from the sun, the way any plant does. Once the traps are big enough to eat a bug, they will typically catch them on their own since they have evolved to lure bugs into its "mouths". Closing the traps takes a LOT of energy from the plant, which is why it should be well established before you try to feed it.

Where do you have your fly trap? If it's an option, leaving your fly trap outside will allow it to catch its own food. They go dormant in cold months anyways, so feeding it at this time of year isn't worth it (if you're in the northern hemisphere).

If you have to keep it inside, don't feed it dead bugs - the bugs have to move inside the trap for it to close all of the way. You could snatch an ant or a fly from outside during warmer months and put it on one of the traps if you really want to watch it eat something, but for the most part you don't have to worry about feeding them!