Whether it’s a line of code or a fancy piece of paper? Your credit card is accepted as money and transfers lines of code saying you gave someone money. You then trust a computer to transfer lines of code from your bank to a vendor. You then pay the credit card off with lines of code of autopay from your bank.
It’s pretty similar.
Note: I don’t buy things with bitcoin, but know people that do.
Federal Reserve notes are not redeemable in gold, silver, or any other commodity. Federal Reserve notes have not been redeemable in gold since January 30, 1934, when the Congress amended Section 16 of the Federal Reserve Act to read: "The said [Federal Reserve] notes shall be obligations of the United States….They shall be redeemed
in lawful money on demand at the Treasury Department of the United States, in the city of Washington, District of Columbia, or at any Federal Reserve bank." Federal Reserve notes have not been redeemable in silver since the 1960s.
The Congress has specified that Federal Reserve Banks must hold collateral equal in value to the Federal Reserve notes that the Federal Reserve Bank puts in to circulation. This collateral is chiefly held in the form of U.S. Treasury, federal agency, and government-sponsored enterprise securities.
https://www.federalreserve.gov/faqs/currency_12770.htm