AFAIK Phoenix doesn't tie in with npm packages whatsoever.
You have an `assets/` dir that contains your javascript stuff, and out of the box Webpack is set up there, but besides that it's all your choice, with the exception of the pre-shipped stuff for Channels and LiveView.
It’s not supposed to tie in with NPM packages. Phoenix handles the back end, and can handle the HTML as well. If you want to include any front end JS stuff, just include the scripts or bundles like you would a regular site where you include JS stuff.
One thing that still felt rough around the edges was the tie in with npm packages. Has that part been improving?