want to build an anti-plagarism software. need help [on hold]

This is a student asking for help! We are trying to figure out how a anti-plagiarism system works. We would like to know if there is a specific algorithm used by all anti-plagarism softwares or if there are different ones for each webpage.

• Do i need a specific algorithm ?


• Any place to find it online?


• Basically how are this free softwares online build?

Your answers will help us develop a sample software for our project at the university. Thank you very much for your help in advance!