Cryptocurrencies are also fiat currencies: the random hashes have no intrinsic value, even less than the paper in a dollar bill. The difference you should be learning about is that in the case of, say, Bitcoin there is no power behind that fiat - nobody is required to use it, pay taxes in it, or receives it without alternatives. That’s why the price is so volatile because at any point a user has to decide whether it’s better to use a different currency (betting that the price will go up).

In contrast, the USD is the default currency in one of the largest economies in the world and millions of people are required to use it to pay taxes, receive payments, or get paychecks from the government, and strong pressure to use it from all of the contracts specifying dollars. That’s a huge amount of inertia and there’s no plausible mechanism where that changes quickly but there’s still a global economy.