How do I look up one subscriber and see their channels and activity?

Use User Explorer. It requires the Personal data permission in your role.

Go to Audience → User Explorer.
In the search bar, choose a criterion: User ID; HWID, Email, Phone; or Token.
Enter the value and click Search, then click the user in the results to open their profile.

The profile is split into tabs:

Tab	What you see
User overview	User-level tags; edit values with the pencil icon
Devices and platforms	Every channel linked to the user (email addresses count as devices), with subscription status, metadata, and tags
Events history	Events, searchable by name and filterable by channel
Received messages	Messages sent to the user across channels
Inbox	What's currently stored in the user's Message Inbox
Segments	Segments the user currently belongs to
Campaigns	Customer Journey entry, status, and exit dates

A Test device badge next to the User ID means at least one of the user's devices is a test device.