What is the Marketing Cloud Enterprise 2.0 account hierarchy, and how does data sharing work between business units?

Enterprise 2.0 (E2.0) is Marketing Cloud's multi-tenant account structure. It consists of a parent Business Unit (BU) and one or more child BUs. The parent BU controls enterprise-level settings and can lock configurations (send classifications, sender profiles, delivery profiles) so child BUs cannot override them.
Data sharing between BUs:
— Shared Data Extensions: Data extensions created in the parent BU can be shared (read-only or read/write) to child BUs via the Sharing settings on the DE.
— Shared Content: Content Builder assets in the parent BU can be shared to all child BUs as global content (locked) or shared content (editable).
— Subscriber data: The AllSubscribers list and Contact Builder contacts are shared at the Enterprise level. A contact unsubscribed in one BU is not automatically unsubscribed in another BU (unless a global unsubscribe is used).
— Reports and tracking: Each BU has its own tracking data; parent admins can view reports across all BUs in Enterprise Analytics.