How to effectively manage user sessions and webhooks in an embedded app?

[fidev]

Hello,

I'm fairly new to Shopify development and I'm trying to understand the best way to address our requirement. Apologies if some of these are basic questions.

The intent is to build an embedded public application that is intended to:

1. Have a floating component that's present on all pages on the online store.
2. React to the user journey e.g. do stuff when the user adds items to cart, completes a checkout, etc. and send events to our server through the journey to allow our server to provide relevant info, regardless of the store theme.
3. Have the ability to do this at an individual session level i.e. not all users will have the same experience.

I had a few questions around this:

1. Will it be possible to add the script to the main theme page and have it load on all pages? Is there a better alternative, particularly if the integration is supposed to be light-touch for admins?
2. What is the best way to get access to the individual user session from the app (assuming we can request the appropriate permissions as a part of the app installation)?
3. Is app bridge and session token required for this? 
4. Is it possible to build this app using Angular? I understand Shopify framework is API-based and in theory any UI framework should work, but will a deeper linking with the user session be possible with Angular?
5. If we get enable web hooks for the various events, would it be a reliable way to detect events happening in the user journey? If so, what will be the correlation id between the events from the app and the web hooks?
6. Is it possible to detect the page the user is in, regardless of the theme? For example: Is there a way to identify that the user has added an item to the cart regardless of the theme used or is a webhook our best bet for those events?

Thanks in advance! 🙂