It's valued from secondary markets based on how much it can raise from private investments. These get reported out eventually which juices the hype further in a bit of a vicious cycle. this cycle also has quite the incestuous component too with how every major company seems to have their hands in each other's pants.

Sale of shares on secondary market at that valuation.

https://www.reuters.com/technology/openai-hits-500-billion-v...