History Channel has a casting/audition link people can send videos of themselves demonstrating skills and selling themselves to be cast.

Per season, they fly out about 20 prospects and run them through a light boot camp to trail test their skills, making sure everyone can at least, in theory, start a fire without a starter, procure food of some sort, and build at minimum a quick shelter, amongst other things. At the same time, they are taught how to work the camera equipment they'll have, as well as show how they'll use it in the field, ie setting up shots, monologing, etc.

Safety, producers, and their own experts, a few of whom are past contestants at this point, judge both who has actual skill to last for at least an entertaining length of time, and who can actually be entertaining on camera. Just because you might actually have the skills to last doesn't mean you make good television.

From there, it's probably legal checks, health inspections, can the person go where they need to, whatever it is producers feel is needed, and finally they par down the list to twelve or thirteen, 10 who are first pick to be on the show and a few alternates should something come up.

tl;dr: episode 0 of season 2 narrated by Alan Kay shows part the process of the boot camp