A good that is sound money doesn't need to be backed by anything. When something is backed by something, it means that some central authority has deemed it so.

When the dollar was backed by gold, that was because the government said it was. But the gold itself is not backed by anything. It has value because it represents an amount of work.

So yeah, Bitcoin isn't backed by anything because it doesn't need to be. The way that it is designed makes it a really good way to store and transact value.

The Bitcoin Standard is a good book for this topic. it's not short, nor a video though.