In real life, people tend to not make huge amounts of money from systems, or get random super legendary stuff.

How would big corp work in a system run world? Would they be unable to have a monopoly? Or would they do something else?

There is no overall system. Each author has their own take on it.

In one story, the system kills everyone on earth except survivors and their cats forced to run a hellish dungeon for entertainment.

In another, you kill giant bugs to get points to buy guns to kill bigger guns to kill bigger bugs.

Or you are given a choice of classes, gain a dozen skills for that class and a dozen general skills. 

Some give 3 stats, some gove 16.

The system in a story is a combination of Laws. Physical laws, magical laws, and government laws. These will define what a corporation can do. 

But going off your original post: Could they have a monopoly? Sure they could. Because almost all systems seem to be based on competition. If you can beat everyone else and create a monopoly, go ahead. But you'll have to enforce your own monopoly the same way you got it. 

It's like someone conquering the world. You can do it, but then you have to fight to keep it. 

Of course, there could be a system enforced rule: Whoever gains 33% of the market share takes over that product. I can make and sell bacon, you can only sell nails. He's got wood, she has sheep, etc.  Doesn't sound like a great system, but some systems are whacky as shit.

The best way to have a monopoly is to have control of a way to make something no one else has or knows about. If no one can make what you are making, no one can compete with you. 

If the companies cannot control the system, then they will control the next big thing: human laws. 

First, they will find someone who can make the best health potions. Then they will make him/her work for them. After learning the secret of the health potion, the original potion maker will either work for them for the rest of their lives or quietly retire. Or get into an accident. Whichever is optimal for the company. 

Then the next phase is to draft a law to copyright the product so that no one else is allowed to make their potion. Anyone who has potions that are not theirs or potion makers who received the skill to make their potions are outlawed or have to work for them. Sure, there will still be an underground sellers, but by then it can never outpace a legal industrialized potion-making industry. 

Then they can sell the product for whatever they want. They can make excuses like massive demand but low supply or some crap, so the price is doubled that month. 

Then they will secretly add a mild permanent poison in the product just enough to be engineered so that every man, woman, and child must take their potion every week or else they die. 

Then they will do it all over again with the mana potion.

If you want the big evil corporation in your fantasy story with a system, then the best answer would make the stereotypical adventuring guild, but make it evil. They control the quests, they are the bank, they control the buying and selling of magical gear and potions, access to dungeons, sale of monster parts, and are international and "politically neutral" (meaning they are too powerful to be intimidated by something as petty as a king) You have them offer the best rewards and benefits to the most powerful adventurers and they use the highest level folks to either press gang up-and-comers into joining, or strangle them in the crib if they will not comply.  Adventuring without a license is illegal and punishable by inclusion on the bounty board, because adventurers without a license are just "bandits."


No more "Adventure's Guild," welcome to the dystopian "Adventuring Co. Inc."  


So other than that idea. The reason Corporations are so powerful in the real world is because money is the power of the real world. Wealth, in durable goods, labor, and demand for service-- those are the things that make people powerful. If you include a system in your world whomever can leverage whatever power of the system will eventually do so and form into a group that can better leverage that system. If that means buying up all the resources or controlling crafting, or even hiring leveled people to be enforcers, then that is what they do. Whatever "power" the system offers, you just have to think how a group of amoral sociopaths would band together to leverage that power into even more power to life lives of luxury and vice. That will tell you how a company will function in a system world. Not talking about doomsday cults out to summon old gods, that's the baddies. Just ask yourself what does the system offer, and what could be done to leverage that power if people were not really worried about morals. That will be the foundation to your corporation, then ask "what if it worked."  That will get you your mega corporation. 

My take on it is that the System and its integration unto society will eventually allow humanity to reach the same cultural and economic summit it has achieved now. 

Of course, there is an argument to be had around whether our current culture is the best Humanity is capable of, but my story doesn't focus on that. It goes of the assumption that it is, mainly because the reader needs something familiar to latch onto.

Hence, big corporation utilize the System in the same way the would utilize weath, connections, etc. From their POV, a good or high class is an advantage in the workplace, but they also keep in mind that every class could be subverted and utilized in certain ways. (For example, two of the reoccurring characters have variations of the rogue class, and are CEOs of a global tech company. Their classes allow them to get rid of the opposition quietly and effectively.)

From an individual's POV, the system is just another factor that influences their chance of sucess, along education, place of birth, potential health conditions, upbringing,...

Quite the opposite. I imagine that an empire centered around one person or group whom became better than everyone else would be the inevitable conclusion of a system run world.

Many systems ive seen feature unlimited instant free transporting of anything bought from the system. That removes any opportunity for companies to move goods from a low value area to were they are worth more simply since the system does that for free instantly.     

Systems also seems to feature banking from a allpowerful incorruptible system entity that outcompetes any human lead bank.      

Most systems seems to feature absolute pricing, when was the last time you saw a system story were a user haggled with the system.     

Basically the system handles so many of the roles companies can handle many system worlds simply dont need any corporations the system handles much of it.       

So what cant the system do what roles are left it cant do that would seem to be what the companies in a system reality would try to fulfill.

I see we have many fellow pessimists in session