Cow's milk is a commodity. It does not make a good currency. It must be kept refrigerated, and even with this becomes useless in a short amount of time. Nonetheless, it is a commodity and is useful.

Gold is a commodity. It makes a good currency - compared to many commodities it is easy and fast to move, can be moved securely, is hard to counterfeit and has a limited supply. It was used millennia ago to make goblets and the like, and has industrial uses today.

Bitcoin is not a commodity. It has no underlying use. So it is not a currency either. Because currencies are based on commodities.

I have written about government fiat money in other posts in this thread so I won't go into that tangent in this reply.