How does an old SANYO camera store proprietary metadata on JPG that are not part of the Exif section? Like if the photo has the correct date or if it was taken using the hand or smile detection or other things like that. Is it an invisible pixel on JPG file like there are invisible Unicode characters? Or is those proprietary metadata stored in the CTG file?
The Exif specification allows for manufacturer-specific data, stored as "MakerNote" data. I'm not quite sure what you mean by an invisible pixel, but there's no need to jump through strange hoops just to store metadata alongside image data.
It is part of the EXIF Info.
Just because you don't see it with whatever you're using to display EXIF Info doesn't mean it isn't there. Very few image viewers even show every Standardized field in the EXIF info. They only show some of them, and different viewers will show different ones. They all tend to show the most common things such as ISO, Tv, Av, lens focal length, etc. But there are many other fields they don't necessarily display.
The Maker Note section is also part of the EXIF Info. The Maker Note section allows each camera maker to put whatever information they wish in whatever form they wish in the Maker Note section. This is very different from the standardized EXIF fields that require everyone to use the same syntax and hex codes for the same information.
Very few EXIF viewers show much, if any, of the information contained in the Maker Note section. Many image processing applications will strip the Maker Note section from the EXIF Info when those applications are used to export an image edited by that application. Adobe products are notorious for this, so of course Adobe products do not display any of the Maker Note section when they display EXIF Info from an image file.