Being born on American soil guarantees you a right to citizenship. Congress allows other persons to become citizens at birth, particularly children of American citizens born elsewhere. That is why many Jews born in Israel to American parents have American citizenship.Congress could limit citizenship to those born on American soil but has chosen not to do so.
The job as president requires one be a natural-born citizen, menaing a citizen at the moment of birth, as opposed to one who is an alien at birth and has been naturalized.
Cruz’s mother is from a family that immigrated to the United States in the 19th century and she is clearly a citizen, therefore her son is a natural born citizen. Cruz’s father was an alien until he was naturalized recently.
Trump is being an ignorant jerk, which might explain why he appears to have very little chance of winning the election (unless he decides to be an intelligent mentsch, which he could easily do as he is an apparently good actor).