What happens to Group files when members are deleted?

I would hope it works this way, but that leads to other questions:

Which user is responsible for the file space used by those files?
A company with 4 users, A, B, C, & D.
Each user uploads 900 GB to their OneDrive, nearly maxing out their 1TB of their allocated space (as per their subscription)

A Group called "Company" is created, and all four members of the domain are added to the group.
The four  use this group onedrive for other files, not stored in their individual OneDrive collections.

1. What is the data limit they can place into the group OneDrive?
   • 100 GB (1TB/user - 900GB used)?
   • 400 GB collectively (100GB left x 4 users = 400GB) and it's first come first served?
   • 1 TB collectively (as implied by OneDrive online documentation), first come first served?
     In this instance, the four users as a company get this 1TB "free," in that no one is responsible for "paying" for it. And if a 2nd, 3rd, 4th, ...nth group is added, would each group get them an additional 1 TB?
2. User "A" leaves the company, and User "E" is hired as a replacement.
   The files in "A"s User OneDrive will disappear after (default) 30 days.
   You say the files "A" uploaded to the group drive will not disappear.
   Against whose 1TB storage limit are these files counted against? Surely it's not free?
   Let's say "E" uploads 500GB to the group, then quits.
   The process repeats for 10 more users, "F","G","H","I","J","K","L","M","N", and "O"
   The org still only has 4 licenses, but it now has 4 active users with 900 GB of files uploaded in their One Drive, plus 15 x 500GB = 7.5TB of files in the group, mostly "owned" by Users who are no longer active.
   This can't be the way it works?

It would seem much more likely that files in groups are the responsibility of the person who uploaded them, or perhaps the most recent person who edited them. And if that's the case, how would an Admin going to deal with a departing employee? What tools are there to handle files that would be auto-deleted 30 days after a User is deleted when the files exist in a Group, not a User OneDrive?

• If they can be located and archived, that's a start... but if it is desired they are not deleted from the Group collection, how can an Admin get them back into the Group collection, preserving the location, permissions, etc. Having to handle potentially thousands of files, and restoring them for use by users who remain cannot be done manually.
• If there is a way to transfer "ownership" en masse, that might help. Admin could locate all file "owned" by departing employee "A" and transfer "ownership" to new employee "E" for example. And by "owned," I'm referring to the responsibility for "pay" for the cost of storage by having the file's existence on the Group OneDrive count against their quota.

I'm at a loss as to which happens, or how to discover it. Fortunately for us, no one is near our quota yet. But I will need to know how to handle this.

Is there a way to find out when and who deleted or removed a person from a group in O365?