From the source
What SynthID is, according to the organisation that built it
Google DeepMind's own description is the only authoritative statement on this and it is short enough to quote accurately. SynthID adds an invisible digital watermark to an AI-generated image, the watermark does not change the image or video quality, it is added the moment content is created across Google's consumer generative products including Gemini, and it is imperceptible to humans but detectable by SynthID's technology. Google operates a SynthID Detector portal and says it is being tested with journalists and media professionals.
One further sentence from that page determines almost everything else here: the watermark is designed to stand up to modifications like cropping, adding filters, changing frame rates, or lossy compression. That is a published design goal, not a marketing claim by a third party, and it eliminates the entire category of amateur workarounds in a single line.