Okay, then where in the Constitution is a citizen defined?
Here is the flow
1) Constitution
2) Article 1 Section 8 clause 4 of the Constitution delegates the authority to Congress
3) Congress enacts Acts to exercise their constitutional authority. This can be seen in the first acts of Congress. Subsequent acts have altered the rules.
4) The current rules are codified as Title 8 section 1401 that defines Nationals and Citizens at birth.
https://www.law.cornell.edu/uscode/text/8/1401