I thought "santa" meant "saint"?
Well, santa is a Spanish word meaning "holy" and saint is a cognate French word meaning the same thing. They descend from Latin sanctus; compare sanctify.
When the prayer goes "holy Mary, mother of god", "holy Mary" is an exact equivalent of "santa María".
[1] https://en.m.wikipedia.org/wiki/Hail_Mary
[2] https://glaemscrafu.jrrvf.com/english/avemaria.html
[3] https://hymnary.org/text/hail_mary_full_of_grace_the_lord_is...
[4] http://www.marysrosaries.com/Rosary_prayers_in_different_lan...
But it makes sense that Church Latin would be different.
https://www.history.com/.amp/topics/christmas/santa-claus#si...
[1] Source: https://boston1775.blogspot.com/2016/12/st-claus-was-celebra...