The claim that "the second you cross into the US illegally, you are a felon" is false. To understand why your claim is false, you must first understand the two different classifications for criminal offenses.
According to Britannica, "misdemeanors are often defined as offenses punishable only by fines or by short terms of imprisonment in local jails" (The Editors of Encyclopedia Britannica Article History). In short, misdemeanors are less severe crimes, and the punishments are less severe.
A felony according to Britannica is, "typically defined as a crime punishable by a term of imprisonment of one year or more" (The Editors of Encyclopedia Britannica Article History). Or, a more severe crime, including prison time, and even a loss of civil rights. However, this varies from state to state.
In Section 1325 of the U.S. Immigration and Nationality Act , "upon first entry into the United States, it is considered not a felony, but a misdemeanor. However, upon re-entry into the United States illegally, you are then tried as a felon.
So, your original claim that “the second you cross into the U.S. illegally, you are a felon” is untrue.