When I came to the United States 52 years ago, Europe was split by the Iron Curtain and even in Western Europe existed numerous border controls. Now you can drive from the Baltic States to Gibraltar without border controls just like driving from Idaho to New York. While Europe tore down the walls, the USA wants to build fences and walls on the Mexican border.
To spend billions of dollars on that is ludicrous. It won’t work.
Illegal immigrants will come as long as they can get a job, however menial and badly paid it might be. Years ago Republicans could have helped making the hiring of undocumented aliens illegal. But no, it was OK with them that farmers, small businesses and private parties could abuse those aliens by hiring them under deplorable working conditions at substandard wages. The inflow of illegal aliens would have stopped if they knew that there was no way to get a job.
Immigrants in general have always been willing to work for a living and have never been a liability for the USA. This country was built by immigrants who because of superior numbers and firepower confiscated the land from the Indians.