>The key thing in monarchial feudalism is that a single person is the state (Imagine a person worth 'only' 14 trillion dollars, he's got his own airforce, navy, army).
This is entirely wrong. Other people have already commented on what feudalism is, but I wanted to provide a brief high-level history in case people were interested.
So let's take the historical look! How did we go from ancient Rome, an imperial monarchy, to feudalism being the law of the land?
The seeds of feudalism were sown from the Roman Emperor Diocletian's decision not to tax lords if all commerce took place within a lord's domain. Diocletian was a wonderful Emperor, one of the best, but he was around at a time when the Empire was cannibalizing itself with constant civil wars, making his main goal was peace and stability. To simplify, if I owned a city, and the city blacksmith supplied everyone else in my city, the state (i.e. the Roman Empire) would not take a cut of this. Only if goods flowed into a neighbouring lord's areas would the empire tax it.
What's the incentive structure here for the lords? Build up as much as you can within your domain and make it as self-sufficient as possible. It's legal tax avoidance, baby!
Because of all the civil wars there were labour shortages, both from bloodshed, devastated landscapes, and people fleeing. What's the solution to this? Outlaw moving, making peasants stay where they grew up and work the job of their father. This made economic planning much simpler, and Diocletian was an economic planner. In doing so, we get serfdom going hand in hand with the seeds of feudalism.
Over time, as the Roman Empire waned, this political structure stayed in effect. Power was increasingly ceded to the regional and away from the imperial. Basically, the state collapsed on itself. While we do get the Catholic Church springing up, it only sorta-kinda fills the power vacuum as it's clearly not a state in the same way Rome was.
Ambitious lords take more power, filling the vacuum of the the monolithic state. Without a single arbiter to enforce relationships between the lords, fealty and swearing allegiances become much more important. Literally the only thing holding these states together were a few lords word, some marriages, and often some hostages. If it was worth it to break those things, the 'state' would crumble, and everyone knew it.
That's basically it. From then, feudalism became the law of the land for around a thousand years (let's roughly say 300AD-1500AD). Then we're onto absolute monarchies, the renaissance, and the bourgeoisie throwing a wrench in everything.
In case all that hasn't helped, here's a (probably weak) analogy: monarchies are monoliths, feudalism is microservices.
FYI, Diocletian didn't use the word lord, he used dux. Which is where we get duke, doge, duce, etc. We also get the word dioceses here, which the Catholic Church still uses to this day.