When I was growing up, textbooks taught me that there were four (or so) distinct "races" in humanity. Caucasian, Negro, Oriental/Asian, and Australoid. We are all the same species (thus, we can mate successfully), but there were four races. Thousands of ethnicities, but only four races.
Later, for political reasons, the people who publish textbooks decided that "race" didn't exist. Seriously. It doesn't exist.
"Racism" is an irrational belief that your own race is superior to other races, or perhaps that one other race is inferior to the other races.
But since "race" doesn't exist - or so we are told - "racism" has been re-defined to refer to anyone who believed anything negative about any other ethnicity. So being an "anti-Semite" is racist. Which is total nonsense, because Ashkenazi Jews are Caucasian. Presumably, anyone who thinks "Mexicans" are inferior is a "racist," even though Mexicans do not constitute any "race" that I've ever heard of.
Trump believes that (a) anyone who comes here illegally is a law-breaker/criminal, and should be treated as such, (b) Most of the Mexicans who come here illegally are the ones who are lacking in education, skills, and intelligence (as compared with the overall Mexican population), and (c) they are disproportionately involved in OTHER criminal activity (aside from breaking our immigration laws). Hence, Trump believes that it would be rational for the U.S., as a matter of policy, to try to remove as many of those who are here illegally as possible. Putting it another way, Trump wants the U.S. to TAKE ITS OWN IMMIGRATION LAWS SERIOUSLY.
How does this constitute "racism"? I don't get it.