Plagiarism Checkers

Ok… so I’ve been submitting essays to a few local contests, and to make sure that most of my work is unique, I’ve been using Plagiarism checkers.
However, I have a question- when you type or upload something into a checker, does your work become property of that checker, or can they keep track of it- or does it simply scan your work and let you know?

I’m quite concerned about this, and I really want some responses.

It does not become their property, lol. Usually, they just scan it and let you know. If its going to keep a copy (I know some school specific ones do), it should ask you first.