I think the externs file maneuver might be outdated.
Admittedly, I'm using shadow-cljs as my go-to compiler (so I'm not sure what the state of vanilla is), but all I do is yarn add <package>, and then require it in the file where I want to use it, and interact with it as if it were any other library.
Though it is true of shadow-cljs as well that it doesn't try to tree-shake external (non-CLJS/Closure) libs by default, though it will do it for project code and Clojure libraries.
Admittedly, I'm using shadow-cljs as my go-to compiler (so I'm not sure what the state of vanilla is), but all I do is yarn add <package>, and then require it in the file where I want to use it, and interact with it as if it were any other library.
Though it is true of shadow-cljs as well that it doesn't try to tree-shake external (non-CLJS/Closure) libs by default, though it will do it for project code and Clojure libraries.