The claim that "the second you cross into the US illegally you are a felon" is false. According to 8 U.S Code 1325, entering the U.S. without permission is a misdemeanor, not a felony, for first time offenders. people caught crossing the border illegally for the first time can face up to six months of jail or a fine but that does not make them felons. law.cornell.edu
It only becomes a felony under 8 U.S Code 1326 if someone was already deported or removed and then reenter the country without permission. In those cases, prison time can be much higher, raging from 2 to 20 years in prison depending on the person criminal history. codes.findlaw.com
Legal experts and organizations like the American Immigration Council also explains that first item level entry is a misdemeanor not a felony. americanimmigrationcouncil.org