TLDR:
The worlds richest man has secretly got the ability to make anything at near-zero cost, humanity is technically post-scarcity. But he sells us things anyway, cheaply, and becomes a monopoly, all governments collapse, and then gives us all charity to purchase things back from him.
He gives a lot of charity, people live very comfortably, but they are being lied to. How long could such a scam last, and what reveals it? After people discover it, what happens to the near-utopia that they enjoyed?
Background
A genius in his garage
A tech genius tinkering in his garage has succeeded in creating the ultimate 3D printer, using nanobots to assemble individual molecules from a stockpile of raw materials into whatever is desired. You can't print gold bars without the same weight of gold raw materials, but you can print perfect counterfeit money by having nanobots assemble the polymers. You can print food using stocks of cellouse and carbohydrates and proteins and arranging them. You can 3D print complex machine parts, computer chips, full computer systems, medicine, anything.
Can't print anything living, cause it moves while printing. But you can 3D print a new 3D printer, even a larger one (if done in peices), and more nanobots for it so it can print faster. The process can't split or merge atom internals however, so no refining plutonium or turning lead into gold.
Rather than announce it, he kept it secret. He wasn't a rich person, and he wanted to profit from it somehow to repay his crippling debts, and pull himself and his family out of poverty.
At first he started an online store selling non-branded electronics. You bought an item, it 3D printed it in it's packaging, applies a shipping label, and a courier collects it. To supply raw materials, and money, he bids for contracts with the city for E-waste disposal. His nanobots dismantle all the cities E-waste down to bland cubes of individual raw materials, suitable for 3D printing new electronics.
His first billion
He then grows to selling everything, basically an Amazon clone. He overtakes Amazon, his company can slightly undercut everyone because the cost is only shipping, and he gets great shipping rates. His food delivery service undercuts UberEats and the like. Many stores worldwide start wholesaling from him because he's cheaper. His garbage disposable business handles the bulk of the western worlds garbage disposal needs. Eventually "Cred" or "Credits at the online store" become a semi-official currency.
He becomes the richest man on the planet, he's basically taken over capitalism and taken over lots of the worlds manufacturing jobs with his secret nanobots. Taxation revenues fall for governments (cause his company is registered in some tax haven and does the same book cooking most big companies do) but he remembers his crippling poor origin, and is filled with guilt. He starts some philanthropy to make himself feel better.
Things heat up
Meanwhile the Earths climate is not looking good. We've totally failed at reducing CO2 and it's getting hotter. A tipping point occurs, stronger and sooner than anyone predicted. Mass methane escapes from the melting Arctic tundra and the planet warms unfathomable amounts in only a few months. Governments totally fail to get organised to do anything. Sea levels rise meters per month. Temperature records are broken weekly.
Our tech genius, senses he can make amends, uses his garbage-disposing nano bots to carve out massive underground civilisation (giving huge amounts of raw material), and his 3D printing nano-bots to fill these massive underground structures with comfortable amenities. They create massive fusion power plants to power everything. They print smart robots to tend to all the needs of people, to maintain things, and he still has his "I can deliver anything to you immediately" service running. People still have access to their Cred accounts, they can still order anything they want from his service, but all banks and governments have collapsed. Our billionare, realising the banking system has collapsed, decides to give everyone a gift of 1000 Cred a day, so that they can continue to live.
(It's very generous - 1000 cred will buy a decent laptop. 5 cred will buy a cheap take-away meal. 100 cred is weekly rent on a nice 2 bedroom apartment. 800 cred is weekly rent on a small mansion. 2000 cred will buy a robot butler that can do all your household tasks for you).
He can't come clean however, so lies on top of his existing lie, and says we're really lucky, cause his robotic manufacturing lines (which people had questioned where his facilities were in the past) are located below ground, so we have access to all the existing supply chains. We can still make computers and couches and hamburgers and sushi and milkshakes. We have all the food we need being grown by robots, and you can still get it and anything else delivered to your door.
Living in luxery
As people settle in, robots are improving the place - growing large parks with fake sunlamps, preparing fake beaches, opening shopping malls staffed by robots, opening nice restaurants and bars staffed by robots, and painting wonderful scenery art so you look out your budget apartment window and see a wonderful view.
Medicine is practised by robots and AI; pills are made with secret nanobots inside it. The AI doctor prescribes you a medication, that medication is 3D printed nanobots. You take it, the nanobots fix you. They do a better job than any surviving doctors can do, they're not getting paid extra so most stop practising, but those that do follow "do no harm", which implies don't do anything because the AI doctor saves more lives than you can.
Engineers are free to create - they can upload a design and have it 3D printed, circuit printed, IC stamped out, etc for a small fee. Many build things which are great, and they want to share them, so they can submit into the online store. GitHub or something similar has survived, and most engineer types are contributing to open source knowledge.
As time passes, people who want to explore are free to do so. Soon people can travel in protective craft up to the surface, and a space program is created. Our billionaire wants to make it to Mars in his lifetime, and plant a new colony for humanity there before he passes. Within a few years of losing the Earth, robots are on the moon and Mars preparing for a colony of humans to arrive, and setting up "The supply chain" to make sure that the colonists will enjoy the same luxuries they're used to.
But it's all a lie
We now have a benevolent dictator, and a post-scarcity world. Lots of energy. Lots of resources. Lots of space for everyone to be themselves. Everyone is living in luxury. Anything they want they can have rock up at their door in an hour. Robots do all the work. No-one has any need to work, they can sit at a computer and create anything they want, or just watch the trillions of hours of video content that was salvaged from the bittorrent network before all the surface datacentres overheated.
People are thankful of the tech billionaire, and every day they get (what they think is) a large chunk of his wealth to spend on themselves, and their lives are bliss. He's peoples hero, and he literally saved the human race.
He doesn't seek praise or fame, no one really sees him the flesh. The billionaire has appointed a trusted successor to take over running his business when he dies - wife, then child. They don't know the secret that we're post scarcity, they believe the lie that there's robotic assembly lines located somewhere else in the massive underground civilisation that are building all the things people want. Most of his day to day work is done by an AI, listing to citizen feedback and changing details or adding options to keep people happy. Approving expansions to the civilisation, adding a new shop to the mall, adding things to the online store, repairing anything which breaks, etc, all done by AI, or AI controlled robots.
No-one realises that he's not actually giving them anything - numbers go up and then down again in their account every day, their rubbish is dismantled, rearranged, and given back to them in a cardboard box with an Amazon-like smiling tick on it.
The lie he told years ago has had lies piled on top of it and has become humanities biggest secret.
How long could this secret last, and what could make it be revealed?
And when it's revealed, what happens to the benevolent dictator and the human race?