The first .NET Blazor "webassembly" was the .NET runtime ported to Webassembly, but where developer code would actually be the same type of IL (Microsoft's Intermediate Labguage) as .NET runs on Windows and Linux.

Unlike those platforms however, the IL was actually interpreted by the ported .NET runtime. Only the runtime was actually running as WASM.

To this day this is still the default, but now you can use a compiler to compile from IL to WASM, and then run full WASM code in the browser. This toolchain is a bit slower on build, but the code will run much faster.