Answer
An admin runs a content search in Microsoft Purview and exports PST, or each mailbox exports its own PST from Outlook.
What it holds
Every mailbox, with threads, attachments and calendars.
Find the size
Exchange admin centre shows mailbox sizes, and the total is on the Microsoft 365 usage report.
What to enter on the form
Microsoft Outlook in GB, and the year your company started using it. Seller form