What is the working principle of the invisible watermark?

Recently, an employee of our company (Alibaba) leaked a screenshot of a computer screen, and the company relied on this screenshot to find the number and name of the culprit. I want to know why this company can rely on invisible watermarks to find information about leakers. And recently, I'm very interesting about the watermarking technology, so I developed a android watermark library that supports the invisible watermarks. the library is still under developments, I need suggestions, thanks.

@link https://github.com/huangyz0918/AndroidWM