Plagiarism detectors aren't meant to be used in lieu of critical evaluation. They find suspected plagiarized passages and supply links to what they think is the original text. Then you as the professor are supposed to evaluate whether it really counts as plagiarism. You can tell the detector to ignore certain parts that pass your scrutiny and rerun the analysis.

When I was teaching, I always included a "drafts only" TurnItIn link that was for the express purpose of students checking their own work for plagiarism. They were supposed to run it through TurnItIn, evaluate what it picked up on, fix whatever issues were present, and then when their paper was in a good place with no plagiarism, they could submit it to the real TurnItIn link for me to grade. This was to the students' benefit because they couldn't be surprised by the results, and also to my benefit because with this system students had no excuse for submitting any plagiarized material. The diligent students used this system. Lots of lazy students did not, and when I found plagiarism I was not lenient because they'd had every opportunity to avoid it. (And also because it's fucking grad school, and I had no patience for their fuckery.)