Domain modeling in the sense of DDD is the primary way fo determining the important aspects of a buisiness. It's usually created collaboratively by engineering, product, and business stakeholders to ensure all major parts of the business are aligned on what the domain model looks like.
Within UML, class diagram can be used to model classes adn model domains.
Entity represents a core concept within the business. Fictional company used in the book is called Streamy, which is a video streaming service.
Each Title belongs to a Genre, and each Genre will have a list of Titles associated with it.
classDiagram
Title -- Genre
In the case of Title and Genre, where each entity is going to hold a reference to the other, that type of relationship is known as an association.
Association is the loosest type of relationship available within a UML class diagram; the entities must be able to exist independently of one another and likely have their own life cycles. Furthermore, there is generally no "owner" of the relationship.
Season and Review do not make sense independent of Title. This type of relationship is called composition.
classDiagram
Title -- Genre
Title *-- Season
Title *-- Review
The side the solid diamond is on indicates the class holding the reference, that is the owner of the relationship.
classDiagram
Title -- Genre
Title *-- Season
Title *-- Review
Season *-- Episode
An aggregate relationship is also displayed as a diamond, but instead of being a solid diamond, it's empty. In an aggregate relationship, there's still an owner - the parent. However, the bond between them isn't as strong, and if the parent were to be deleted, the child can still exist.
classDiagram
Title -- Genre
Title *-- Season
Title *-- Review
Title o-- Actor
Season *-- Episode