Most often the Immigration and Customs enforcement (ICE) identifies likely illegal aliens via fingerprints. When local law enforcement takes someone in custody, the inmate is fingerprinted. The fingerprints go into a national database, which is automatically available to ICE. If ICE identifies the person as a likely illegal alien, they will issue a detainer requesting the local agency to hold the person for a couple of days or let them know when the future release date is, so they have a chance to pick him/her up. Sanctuaries only honor such detainers for certain more serious crimes.
They are harboring in that way other illegal aliens, who have committed only more minor crimes, protecting them against federal law enforcement. Aiding and abetting illegal aliens is, itself, a crime, a felony. However, the law is being ignored for politicians.
California is the biggest problem state. They have been a sanctuary state since January, 2014 when the Trust Act took effect. “Trust” is used here as a euphemism to mean “sanctuary.”
Leftists love to play with words to obscure things and confuse the public. Recently, Leftists have been trying to redefine the term “sanctuary,” so that places, which used to be considered “sanctuaries” are no longer called sanctuaries.
If a city, county or state is not cooperating fully with ICE they are still a “sanctuary,” no matter whether they call themselves that or not.
Obama had a policy of not issuing detainers for illegal aliens charged with minor crimes. So, in that sense we have been a sanctuary nation in recent years. So, cities could also claim to not be a sanctuary, because they had not rejected any detainers from the Obama administration. Hopefully, that policy will change.