All Subscribers is a view that is shared by all Business Units in your account. A Subscriber added to All Subscribers in Business Unit A will typically be visible in the version of the the view visible in Business Units B and C along with all the profile attributes populated in Business Unit A.
The Status field in each Business Unit (the field that determines mailability and is updated through unsubscription) is a little more complex and is specific to each Business Unit. Depending on the Business Unit's settings, you can specify whether unsubscription should apply to just the current Business Unit or to all Business Units. The setting can be found in "Email -> Admin -> Business Units".
There's a deprecated feature called Business Unit Filters that used to allow you to select just a subset of All Subscribers that may be visible in a particular Business Unit. Few accounts have this feature available these days.
You'll find that access to All Subscribers is frequently blocked in many accounts to disallow users from Business Unit A from seeing Subscribers from Business Unit B.
And likewise, there's a permission to disallow viewing of All Contacts in Contact Builder