The rest become part of a tragic surplus of pets. Most of them die. They get sick. They starve or freeze to death. They are hit by cars, become wounded in fights with other animals, and suffer abuse by uncaring people.
Some kittens survive to breed and start colonies of free roaming cats.
Some of the fortunate animals end up in shelters, where trained caregivers make every effort to find them new homes. But the numbers tell a tragic story. There are many more pets in shelters than there are people able to provide permanent, responsible, loving homes. This is a heartbreaking waste of a life. The real solution prevents the surplus from occurring.