Usually Bitcoin is measured in dollars to increase the value of its investment because the dollar is one of the most popular and powerful currencies in the world, but you can choose your own currency if the dollar is not your currency. There are many choices on coinmarketcap.com.
That is just the price of bitcoin as per the fiat vs bitcoin trades happening in the market. It does not give any sort of inference that so much bitcoin is backed by so much fiat. Because that is not the case. Bitcoin itself is independent but for monetary and trading purposes we need to set a market capitalization and a price for common people to buy/sell it as an asset.
Think of bitcoin as something like a currency but not directly linked to fiat. Its independent but need fiat to be transacted. That is for the time being till bitcoin takes over fiat.