Although there are many open source options available to check for plagiarism such as scribbr, PlagScan, OpenSource Plagiarism Checker (OPL) and grammarly but universities, journal editors and databases prefer Turnitin as 'gold standard' for both similarity and AI detection.

One common weakness to open source detectors is limited access to databases i.e. may not compare submissions against as large a database as Turnitin. Additionally, these are not always 100% free.

Hope this helps. If you want to check your documents for AI or plagiarism through turnitin, I can help you. Thank you :)