iCloud Photo Library on multiple devices: how to keep a centralized location for originals? Say you have 3 devices MacBook, iPhone &amp; iPad. 
You start storing all your original photos on the MacBook, as you want this to be the centralised location. iPad &amp; iPhone are configured for "Optimised Storage".

As soon as you start shooting pictures with your phone, originals will be stored on phone and MacBook. Or images are uploaded from phone full quality and then downloaded to MacBook as unmodified originals and left on phone as smaller miniatures?
When you shoot something with the iPhone, it is full sized only if 


It is recently clicked or opened. 
It has not been uploaded to iCloud. 


When iCloud gets the full size image, and iPhone gets low on storage, image is downsized.

When MacBook syncs the photos, it downloads the full size image and keeps it. 

When iPad gets the photo, I don't know what happens in transition, but ultimately, it has only a thumbnail.