Do all undocumented immigrants or illegals sneak across the border?

///Do all undocumented immigrants or illegals sneak across the border?

No.  Although accurate numbers are hard to come by, approximately 40% of the over 11 million undocumented immigrants in the US originally came to the U.S. legally.  For example, as tourists  or some other non-immigrant or temporary visa.  After the time expired for their stay they didn’t return to their home country.  The moment they failed to depart they became “undocumented.”