[unlearner]

“Isn’t ‘at birth’ when you said it happened? Congress says it is.”

Naturalization is the process of becoming a citizen through a process specified by law. It CAN be based on when and where someone was born, but it does not make applicable people retroactively citizens at birth. So, for example, Congress amended the Constitution to insure that blacks who resided in the US were unquestionably citizens. They did not have to apply to become citizens, but they were naturalized.

[philman_36]

A VIEW OF THE CONSTITUTION OF THE United States of America. BY WILLIAM RAWLE, LL.D.

CHAPTER IX. Of the Enumerated Powers of Congress

The mode by which an alien may become a citizen, has a specific appellation which refers to the same principle. It is descriptive of the operation of law as analogous to birth, and the alien, received into the community by naturalization, enjoys all the benefits which birth has conferred on the other class.

I hope those help you better understand what that phrase means.