The use a library called 'libcosmic' that uses Iced framework that is written in Rust.

libadwaita/GTK4 is to GNOME what libcosmic is to COSMIC.

COSMIC has no need for an alternative to GLib or GIO. Rust's standard library, the futures crates, and tokio are more than sufficient for handling I/O and asynchronous code execution. libcosmic uses tokio executors for handling application commands by default. You can bring your own executor and thread pool libraries if you want.