How Does WebAssembly Work?

In Bitcoin, you define your blockchain in some code and then that compiles the WebAssembly. This is your runtime. But what are these run times actually doing? Like how does this WebAssembly piece work? With Ethereum, most of it remains the same. All of the stuff that I just described, synchronization, strategies, working out whether it's the canonical chain or whether it's something that's spelled by the wayside,. It's all the same, pretty much. There are one or two minor details that have changed, but it's very much the same thing. In Bitcoin, to take a simple example, what it means for some transaction outputs to be spent is that some balance has