Literally every type of money throughout human history was unit of SOMTHING - cow, tobacco, metal, and, currently, debt.
Correct.
But if I were now to ask you: "unit of what is Bitcoin" what you would do is offer some generic answer like this: "Bitcoin is a coin, a digital asset, a currency, money, resource."
If you asked me "unit of what" is Bitcoin, I would explain that it stands as a unit unto itself, not tied to anything. For those lacking creativity, it might be thought as a number in a database.
I can write down or put in some database the following: "Bob has 10 units of ABC". Then someone asks: what is ABC? As a response I offer the following generic answer: ABC is a digital asset. Then they ask: "What actually is that digital asset? What it can do?
Your ABC number would hold no value, and furthermore, it couldn't have any real worth due to its absence of essential monetary attributes. It lacks durability and a stable monetary policy. You could generate infinite ABC coins by merely altering your spreadsheet. If your ABC number possessed durability and a verifiably fixed supply, it might begin to be regarded as a form of money.
What appears to me, is that you fail to understand that money doesn't necessarily require backing by a physical asset. In ancient times, when tobacco served as currency, its value didn't stem from its tangible real-world worth per se. It was accepted as money because it fulfilled the essential characteristics of currency. Same for gold.