

This is not the same for all crypto currency, but a bitcoin represents a “proof of work”. When people “mine” bitcoins, they are consuming computational resources, and when they find a bitcoin, it is a certification of the work that was done to find it that becomes the value of the coin. And then, as others as mentioned, people just agree that that work has a certain amount of monetary value. But the proof of work is what limits the supply and allows that value to exist. 3Blue1Brown has a really good video that goes into the technical details if you’re interested.
If you don’t care about the digits of pi being in base 10, there’s also a formula that can be used to calculate digits at any position you want in hexadecimal without having to compute the preceding ones!