In industry there are breeder companies.
Females are selected for size AND egg production, while the males are selected for size alone.
These lines are brought together when the company is ready to breed the broiler (cornish crosses) that we eat.
When the two lines are crossed, Hybrid Vigor is seen and the birds grow VERY fast and mature in 8 weeks...in time for processing.
The BREEDER birds are kept on a strict diet so as to obtain as many eggs as possible (160-260/hen).
In essence...the breeder birds are separate breeds than their offspring. It's confusing...I know...LOL. They breed naturally...no AI involved in these birds.