The term “plagiarism” has different meanings, but it usually includes copying somebody else’s work without permission.
In my opinion, most of the free-software-checkers for plagiarism aren't safe and don't work effectively. Unfortunately, you have to pay for the sake of getting good results.
There is no guarantee that the original content of your manuscript might not be copied and sold to others before it is published by you. So, I discourage using any free-software-checkers for plagiarism; some of them are betrayers. Please don't forget that:
There are a massive number of betrayers!
Plagiarism is a sin! If it is, severe action may be taken and/or legal notice may be given.
In any way, it is not well for your reputation if there were accusations of plagiarism. This is because plagiarism is a sin! If it is, severe action may be taken and/or legal notice may be given.
There are many plagiarism detection software are avilable. Among them, the best one, I think, iThenticate and the second one is Turnitin. However, these softwares are not freely avilable. You have to purchase these softwares. Alternatively, you may use plagscan, detector.net. etc.