For example, several people have no money, but want to play a few games of mahjong, and then make an appointment for a card of poker to represent one dollar. Poker then generates a dollar value. The value it carries at the moment is the mutual promise of several people, and its function is to calculate the quantity.
In today's digital age, the immutable properties of the Bitcoin database accurately carry the mutual promises of Bitcoin holders, which can quickly calculate the quantity and transfer each other.
As a final note, Bitcoin is hard to come by, and if 21 million people buy one each, they will never be able to buy it again.
Bitcoin is the name for a unit of a number that Satoshi Nakamoto created in their imagination - 21 million. This can't be valuable by definition given no one can utilize a number produced in someone's imagination. I mean, even if they could, everyone can use their own imagination to create whatever number they want. Bitcoin is simply a bait to scam people out of their economic resources, because it is these resources what provides utility to people. All scamers want to get their hands on those.