Looks like this runtime is written in Rust. Really does seem like Rust is rapidly swallowing all kinds of common tools and libraries. In this case a single compiled binary for multiple architectures is quite convenient for something like yt-dlp.

Deno itself is written mostly in Rust, but it also leverages [1] Google's V8 Javascript engine which is written in C++.

[1]: https://choubey.gitbook.io/internals-of-deno/architecture/v8