1- Does it download all the files for that specific build or does it download only the files that changed?
2- I’m seeing a bug which I get a white screen on my iPhone 6 (using ionic 3) when I select auto for the UPDATE_METHOD variable, is it normal? I had to change it to be background because of that.
I know, these are kind of questions/observations/feedbacks points… But I’d love to better understand what’s going on 
Ionic Pro is a hosted and closed source service, so there is not much we, the community, can help you with.
There is a FAQ and common/known issues at https://ionic.zendesk.com/hc/en-us/sections/115003254647-FAQ-Common-Issues
Official documentation is at https://ionic.zendesk.com/hc/en-us and https://ionicframework.com/docs/pro/
You can - and should - contact support at http://ionicframework.com/support#support
As far as I know, the whole app as a .zip file, including the /assets folder. Can get quite big.
No experience with that. You should contact support about that.
Thank you, sir!
(few more words here so that I can post the message ;D)