I think you would need to see these boarders in person to really understand.
I have crossed the boarder from Texas into Mexico from Larado Texas using just my Driver’s license, this being back in 1980’s and some on the 1990’s , today you have to have a passport.
Had I felt like crossing the rio grande river, I would have just walked around or climbed over the fence and entered. The fences and walls only run so far, actually not that far at all.
I have also crossed from SanDiego California into Tiajuana Mexico with nothing more than my driversi felt license back in the day.
Had I felt like getting wet, I just would have taken a very short swim in the ocean around the boarder fence and entered that away.
If anyone in this thread really desires the truth of the situation, Come down to Texas, I will be happy to meet you in Larado Tx, Brownsville Tx, McAllen,Tx , El Paso Tx and show the truth.
I stayed a week in Monterey Mexico with a friend of mines family, no passport.
I have been to Juarez, Monterey, Reynosa, Tiajuana Mexico