How does Contact Builder work, and what are attribute groups, attribute sets, and populations?

Contact Builder is the identity resolution and data modelling layer in Marketing Cloud. It links data from multiple sources into a unified contact profile.
Attribute Groups: A named collection of related attribute sets (DEs) that share a relationship to the contact. For example, an "E-commerce" group might contain Purchase History, Product Views, and Cart Abandonment DEs, all linked to the contact via ContactKey.
Attribute Sets: Individual Data Extensions linked into Contact Builder. Each attribute set must be linked to a contact via a shared key field (e.g., ContactKey, Email).
Populations: Named groups of contacts with common characteristics defined by a filter. Types:
— Individual: One contact per person (most common).
— Household: Groups contacts by a household key.
— Account: Groups contacts by an account/company key.
Contact count vs Sendable contact count: Contact Builder counts all contacts in the system (billable); sendable contacts are those with a valid email address on a sendable DE who are not globally unsubscribed.