This example is based on this tutorial how-to-setup-a-phoenix-and-clojurescript-project. It shows how to set up a Phoenix LiveView Elixir project for development with ClojureScript, which then can be used to write Hooks. Here this is alluded to in the app.main.hi() call in assets/js/assets.js. The ClojureScript could sits in the assets directory. Via shadow-cljs you have the usual hot-code-reload support.
The example also adds a scss pipeline, based on this article. The author proposes to commit the generated artifacts (here priv/static/assets/assets.css) to the repository so that a production build can use them.
$ mix deps.get
$ mix phx.server
visit localhost:4000