Tauri is "just" a system webview wrapper. It's an interesting alternative to Electron for some applications because the installer is much smaller, but it also has downsides compared to Electron (the system web view will be updated independently and may unexpectedly break the application).
As far as I understand, applications are not written in Rust but in HTML/CSS/JS.
There's a communication channel between the Rust code and the JS running in the browser. It's possible to move as much functionality to Rust as you want... It's even possible to compile Rust to WASM to use that in the "frontend" though that from my experience is very cumbersome due to the Rust bindings for the Web API being pretty terrible.
As far as I understand, applications are not written in Rust but in HTML/CSS/JS.