Humans needed to eat food to survive and function as living beings. The energy in their body was broken down from carbohydrates, fats, etc.
In the Darble World, there was the concept of Darlac, an energy source that was the condensation of one's stamina. It could be said that Darlac was the primary energy of the Darble World.
And in the apocalypse world, after the apocalypse, it was the Apocalypse Pearl.
In a cultivation world, it was Spirit Energy. And finally, in a magical world, it was Mana.
Basically, every living being relied upon some energy type. And, this energy type was something that they were naturally accustomed to dealing with.
Consider Darlac, for example. It was the condensation of stamina. Every flesh and blood creature had stamina. And, stamina was something that could be replenished through rest, eating food, and drinking water.
For creatures in the Darble World, they were naturally equipped with the means to generate, wield, and replenish Darlac.
If you take something like a golem, it wasn't capable of generating, wielding, or replenishing Darlac since it wasn't equipped with the means to do so.
In that sense, the energy medium used by Systems was Mental Energy. That means, they were equipped to generate, wield, and replenish Mental Energy. It was what they lived, breathed, and ate to survive.
And, it has to be noted that Mental Energy was nothing beyond the thoughts produced in the mind of any creature with the faculty to think. Thoughts condensed were what was termed Mental Energy.
Further going with the thought, not only do Systems eat Mental Energy, but they are also able to understand data from Mental Energy. This was how they find out information related to the people in the world they are in.
But, this was just in terms of consuming Mental Energy. Here came the actual question then.
Why did Systems pick a Host to live within? Why did they do that? Why do they give Quests to their Hosts and also reward them upon completion?
The answer was simple once he thought about it from a different perspective.
Pollen!
Flowers contain nectar that bees suck out. And in this process, the pollen from the flowers get stuck to the fur of the bees that are then transported into the flowers that the bees move towards. This way, it helps fertilise the flowers, causing them to bear fruits.
In this analogy, the System was the flower, and the nectar, the Quests that attracted the bee. But that wasn't all. When the bee interacts with other bees, the pollen would rub onto each other and be transferred to other bees too.
And, these were the people the System Host interacted with, with him/her being the bee that drunk the nectar.
Now, what do the pollen help with? Fertilisation, resulting in the production of a fruit in which seeds are contained. And, these seeds would one day germinate into a sapling and end up growing into a tree.
"That's why a System's Core Skill doesn't vanish. It is indestructible because a System's true body lives in the mind of everyone with the knowledge of it. That means…" Compass Carburettor turned around, snorting, "Come out, Money-Making System. You're the one that I thought about the most. So, you should be condensed in my mind the most."
"As expected of my previous Host, you're brilliant. I was right, you mature the fastest under pressure." The Money-Making System's figure appeared as it let out a bloody grin, "That's right. Our true body lies in the minds of everyone we've come into contact with. Or in a simpler sense, everyone that has thought about us harbours a fraction of us in them."
"And that's why your Core Skills will never truly be mine, since I haven't actually refined them. Right, Directional Misalignment, Info Scanner, Reality Marble, and Slave Harem?" Compass Carburettor turned around, watching the four respective Systems appear around him.
It was surprising because he noticed that he was in control of his emotions now, "Humans only fear the unknown. And now, your truth is no longer unknown to me."
"Even then, you cannot do anything to us." The Money-Making System smirked as a bullet from Earth passed right through its body. The energy blade from the Darble World too passed through it, same for the Dust Wave from the Apocalypse World.
"That's a great way to taunt me, to be frank with you." Compass Carburettor smirked as he arrived before the Money-Making System, gently placing his hand on it, watching his hand pass right through, "We are unable to interact. So, this implies that even though I can't do anything to you…"
He stared right at it, grinning, "You can't do batshit to me either."
"You…!" The Money-Making System grit its teeth. And suddenly, it was shocked to see Compass Carburettor break into laughter all of a sudden.
"Mental Energy!" He cackled, "As beings that eat that, it's impossible for you to not feel any emotions, since the thoughts of every living being are rich in emotions. The act of you being emotionless before was pretty great. And even if it was real, nothing changes anyway."
"I figured out something. If you all are living in me from the moment I thought about you, why haven't my plans been leaked to you yet?" Compass Carburettor pointed at the Slave Harem System, "Especially you, Slave Harem. Even though I have blocked my thoughts from flowing towards you, that doesn't mean I wasn't thinking about you. Rather, most of my thoughts were about ways to kill you."
"Let me answer myself," He coughed, saying, "It's simple. You're part of me. Or rather, you're parts of your respective Systems that are trapped in me. And, some conditions need to be achieved before you're able to leave my mind and return to your original body. And, your Core Skills are something like a compass that is linked to all your body fragments scattered through the minds of everyone."
"And, your actual growth isn't through Mental Energy. That's just what you consume as food and to activate your powers. Your power grows stronger by absorbing these fragments that are growing in the minds of everyone." He smiled, "How's that?"