Why class is known as a composite data type?

Answer 1

And pls mention the language you are talking about, either in the sentence or in the tags

Answer 2

It is because of its composition, it consists of instance and class variables, methods and references to objects of other classes etc.

https://en.wikipedia.org/wiki/Composite_data_type

Also Composition is a technique which implements the HAS - A relationship in classes.

class Employee {

int id;
String department;

}

Employee has-a id.
Employee has-a department.