Amazon: people like books; people like next day delivery of stuff; people and companies like making stuff and running stuff in Amazon web services
Minecraft: Marcus Persson owned the game studio (and wrote quite a bit of the game) that made Minecraft, lots of people like it, Microsoft was willing to buy it for billions
Kiran Mazumdan-Shaw made beer, people like beer. They then used beer making processes to make biotech medicines - people like being alive and will pay a lot to stay alive, or even just a bit healthier
Is that how value is determined? How others perceive value? Isn't that kind of subjective? Is there a possibility he isn't worth that much money? Could he be worth even more? Not like margin of error, but dramatically more or less like 20-30% off one way or the other?
The value is tecknically the effect of what is called a stock market, and it does vary quite a bit over time as perceptions and economic factors change