The Scribbr AI Detector is based on the same principles as all AI detectors, also the Turnitin AI Detector.
AI detectors measure specific characteristics of the text (perplexity and burstiness). With this, they can identify whether a text is potentially AI-generated. When text contains characteristics that are similar to text written by AI, your document might have a higher probability of being written by AI even though you wrote it yourself. Examples of these characteristics are...
Text that reads monotonously, with little variation in sentence structure or length
Text with predictable, generic word choices and few surprises
You can find out more about how AI detectors work here.