If you parse the webhook event in step 6, you will get the recording URL to download the mp3 file. However, to actually download this file, you need to authenticate to the Zoom download server to actually download the mp3.
So I would recommend the revised step below:
Setup a custom endpoint listener
Setup Zoom App
In Zoom webhook - “Call record ending” point to the custom endpoint
Make a call to the zoom phone
Call details should be send to custom listener
In the custom listener, validate all the details
In custom listener app, using authentication ( JWT or oAuth ) download mp3 file from Zoom cloud.
In custom listener app, create phone call object and mp3 file data to hubspot.
I am not sure in step 8 whether you can upload mp3 data to hubspot as an object. If hubspot doesn’t support this, you will need to host the mp3 files on another web server somewhere that can be used to store the data and you would need to handle authentication to make sure that only authorized users download/access the call recording mp3 file.