They're not backed by anything, you obviously don't know what fiat currency is.

USD was once backed by silver, as were British pounds. They are no longer "backed" by anything other than what people are willing to trade for them. Likewise, Bitcoin is worth whatever people are willing to pay for them. There is no difference other than the fact that the market caps and stability of USD and Pounds far, far higher than Bitcoin.

You fail to explain how Bitcoin is any different, and your claim that stocks can't be exchanged for good and services is completely false— people literally take stock options as part of their income at many companies, and there are definitely people out there who would gladly trade you goods or do you favor for valuable stocks. It's not as common as exchanging for currency, but there is no inherent difference.

The thing that all these things have in common is that their value is not derived from their physical properties. It's really not a hard topic to understand, and honestly it sounds like you're just regurgitating something you read without understanding what you're talking about.