a category
A group that shares similar characteristics but little else is known as a "cohort." Each of the examples you provided describes a specific cohort based on shared characteristics such as gender, handedness, or height.
Mongoloids look the same because they share a lot of the same DNA traits and characteristics. This is the same reason why people with Down Syndrome all look similar or why little people all look similar.
The literary term that describes "birds of a feather flock together" is an idiom. This expression conveys the idea that people with similar characteristics or interests tend to associate with one another.
A category is a group of people or things with shared characteristics or features.
People have similar characteristics, including religion, age, race, income, body type etc.
Equality of condition describes a state in which people have a similar amount of material wealth or in which the general economic conditions of their lives are similar.
A factory where people make very little money for hard work
Market segmentation
Many Filipino songs have similar characteristics. They are often about love, feelings, and the emotions people deal with when losing a love.
Social homogamy is the tendency for individuals to form relationships with others who are similar to them in terms of social characteristics such as education level, occupation, income, and social class. This phenomenon reflects the idea that people are often attracted to those who share similar social backgrounds and values.
The Chinese people all look similar because they share common DNA and genetic characteristics. They stem from a common group of interrelated ancestors and have inherited their traits from them.
A factory where people make very little money for hard work