Several AI-powered plagiarism checkers are available, each with its own unique features and capabilities. These tools analyze text to identify similarities with existing content across the web, academic databases, and other sources. They provide detailed reports highlighting potential instances of plagiarism and help users ensure the originality of their work. I will suggest you to check this plagiarism checker tool which provides best results as compared to other tools in the market as per my research.
As far as I know, institutes use different sites where they have their own database of all students and AI compares and writes their percentages of identity.