Generalization, Specialization, and Aggregation are mainly used to hide details of a set of objects or these are used for data abstraction. Generally, in any system, there are three layers, the first layer with which user interaction is known as the application layer, and the last layer with all system’s complexities and data is known as the physical layer. There is another layer in between that hides the system’s complex data from the user known as the abstraction layer or logical layer.

So to perform data abstraction or hide the system’s complexities from users requires three mechanisms, known Generalization, Specialization, and aggregation.

Generalization: Generalization is the process of creating a generalized entity by collecting common properties from a group of entities. It is a bottom-up approach. In this mechanism, two or more entities can be generalized to a higher-level entity if lower-level entities have some common attributes. For example, a person is a higher level generalized entity for Student and Teacher.

Specialization: Specialization is a process in which an entity is divided into sub-entities based on their properties. Specialization is a top-down approach in this a single higher-level entity can be specialized into two or lower-level entities. When lower-level entities have some different attributes.

For example, an Employee is a higher-level entity in any employee management system, which can be further specialized into Tester and Developer or further profiles.

Aggregation: An ER diagram is unable to represent the relationship between an entity and a relationship. Aggregation is a process of applying abstraction to represent relationships as higher-level entity sets. Wherever a connection between an entity and a relationship is required, in those cases, relationships with its entities are aggregated into a higher-level entity.

Generalization, Specialization and Aggregation in ER Model: https://www.geeksforgeeks.org/generalization-specialization-and-aggregation-in-er-model/