So that makes monopoly money different than regular money? It's that monopoly money is really easy to reproduce, no one will stop you.

With government backed money the thing stopping you is the government. They arrest you if you try.

With gold the thing stopping you is the ability to dig out more.

With cryptocurrency the rules are enforced cryptographically through any number of different algorithms. That basically means that more or less anyone can create a bunch of rules for how their new currency can be used, traded, etc without needing to be a government.

The trick then is convincing other people that your idea is worth it as a currency.