There are photoshop plugins around like imatag or digimarc, which offer this as a professional service.
The software encrypts a bit of information in a bit pattern and then hides this pattern in your image numerous times. This will work ok as long as there is enough variation in the image, which usually is true for photographies. The information is somewhat ...
The Nikon focus points map like this. What that means in terms of image pixel size IDK... a lot more than 1.
FWIW, this is nothing like how the AF sensor/sensor lines actually look/work.
The exif for a D850 image reads like this:
AF Area Mode : Group Area
Phase Detect AF : On (153-point)
AF Points Used : E12
Primary AF Point : C12
Which means: Group ...