which of the two options do cryptocurrencies use?

Fiat is just mainstream legal tender, or official national currency, issued by governments . This includes the US Dollar, Canadian Dollar, Euro, Japanese Yen, etc. Fiat isn’t backed by any commodity (so when the USD used to be on the gold standard, it wasn’t fiat).

Fiat has value by decree of the State. Ethereum, like Bitcoin, gold, and pork bellies, has value by bids and offers in an open market. You can hate on ETH for a number of reasons, but it ain’t fiat .”

Bitcoin and fiat currencies are not backed by any other asset .

Unlike fiat currency, most crypto is entirely decentralized and operates peer-to-peer without any intermediary . Some cryptocurrencies operate on private ledger systems that are controlled by a single entity. Cryptocurrencies are generally backed only by the faith of their users.