We can explain it for you, as we have, but we can't understand it for you. I'm really not sure what you aren't getting about it being racist to tell native-born U.S. citizens of three different non-white ethnicities to "go back" to the countries "from which they came" (but did not). I mean, it's racist for multiple reasons, including the implicit notion that non-white citizens cannot be true Americans, and the long, history of the phrase being used with racial animus to target minorities. And that includes Omar, even though she wasn't born here.
But even at the most superficial, semantic level, the definition of racism includes "racial prejudice or discrimination" (https://www.merriam-webster.com/dictionary/racism). Trump's false assumption and assertion that 3 native-born women of color "originally came" from other countries is racial prejudice in its most literal sense.
I know Republicans have made their bones on using sophistry to hide racism under pretext, but sometimes a particularly stupid Republican goes so far that you can't hide behind the disingenuous claim that, "durr, you can't prove to a certainty that he thinks whites are superior to other races or that he was prejudging someone based on their race!!!" But Trump is particularly stupid and said the quiet part loud this time.