Is it possible to allow a user to upload a video to their channel through a HTML/javascript form? If yes I am not sure how the google authentication would work on users end. The code snippets refer to using one's own account (e.g. api key and secret key), so they are not helpful in my case.
It's possible, but you need a backend/server-side code too and wrap up your form as an app and the user needs to allow the app to be able to upload the video through your app on Youtube.