This will import hello into the scope of app.js, but will not make it available to the HTML document automatically unless you assign the function to the global window element.

// file: assets/js/app.js
import {hello} from "../vendor/my_functions.js" 
window.hello = hello

A good next step would be to namespace your exported functions, then assign that to the global element:

// file: assets/vendor/my_functions.js
export function hello () {...}
export function etc () {...}

// file: assets/js/app.js
import * as MyFunctions from "../vendor/my_functions.js" 
window.MyFunctions = MyFunctions

// in HTML <script>
window.onload = function () {
  MyFunctions.hello()
  MyFunctions.etc()
}

This can start to get unwieldy when you have a lot of functionality in multiple HTML scripts.

I would instead move them to app.js and set up your code from there:

// file: assets/js/app.js
import * as MyFunctions from "../vendor/my_functions.js" 

window.onload = function () {
    MyFunctions.hello()
}

// now this is only necessary if you also want to access them directly from the dev console
window.MyFunctions = MyFunctions

Lastly, if app.js also gets too large for your liking, or you only want to have certain features running in specific pages, you can move the relevant code to its own module, have esbuild compile it separately, then load it into your page.

// file: assets/js/my_feature.js
import { hello } from  "../vendor/my_functions.js" 

button = document.querySelector("#someButton")
button.addEventListener("click", () => { hello() }

add js/my_feature.js to the esbuild args string (and restart the server to apply changes to config files)

# file: config/config.exs
config :esbuild,
  version: "0.17.11",
  default: [
    args:
      ~w(js/app.js js/my_feature.js --bundle --target=es2017 --outdir=../priv/static/assets --external:/fonts/* --external:/images/*),
    cd: Path.expand("../assets", __DIR__),
    env: %{"NODE_PATH" => Path.expand("../deps", __DIR__)}
  ]

<!--- my_page.html.heex or inside render/function component ~H template -->
<script defer phx-track-static type="text/javascript" src={~p"/assets/my_feature.js"}>
</script>

<button id="someButton" type="button">Run hello()</button>