Free plagiarism checkers are high risk for your thesis as shown by Anton Vrdoljak They are only good for checking short texts, such as a paragraph as recommended by Ondrej Pelech, for example if you are checking on students' work for plagiarism.
From my point of view, I don't recommend you to use such free software checkers of plagiarism.
I haven't used them before because I am afraid that the original content of my manuscript may be copied and sold to others before I can publish it.
I know that the problem is not easy for us, especially that we haven't enough money to buy enough food and, in turn, to buy such software. So, I always write using my own words.
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.