No, you cant just walk into a Walmart and they give a job after talking to you in a back room. There is no way a corporation would risk that. They have to find a broker. It's not that easy. They want people who will keep their mouth's shut.
These jobs are not advertised. You can only find about them from word of mouth. They come from agricultural and construction industries. American contractors hire legal immigigrants to do these jobs as subcontractors. The subcontractors then hire who they want to.
This has gone on for decades. The contractors like it. The illegals like it. Hell, it makes produce cheaper.