Why is Baja called Baja, California when it is in fact in Mexico?

"California originally referred to the entire region composed of the Mexican peninsula now known as Baja California and land in the current U.S. states of California, Nevada, Utah, Arizona, and Wyoming, which was eventually distinguished as Alta California."

So originally there was Alta California and Baja California - Baja went to Mexico, Alta to the US, and the "Alta" was dropped from the name of the northern area. Also, Alta California covered a much larger area than the modern state of California.

More here:

https://en.wikipedia.org/wiki/Origin_of_the_name_California

California was originally part of Mexico. "Baja" California is "Below" California