It works more or less the same like copyscape whereby it checks whether there are similar syntax from the same content appearing at other sites. Usually it will easily tell you whether it has been plagerized
These app's are good, but in the end I have used a few and each one has shown 1 or 2 sites who ripped off my content and the other 900 just look alot like it but the content itself has nothing to do with my original content lol
Yeah, that's possible because basically some of them use algorithms that check for probability to determine whether it might have plagiarized. It's not 100% accurate but the surefire method is by looking at one particular sentence and finding the exact ones that appear on other sites.