How/where does a GoPro camera store HiLight tags?

Solution 1:

I’ve found the HiLight tags: they are stored in the MP4 files themselves.

In particular, the tags are stored in a box with type HMMT in the User Data Box (udta) of the Movie Box (moov) of the MPEG-4 container. See ISO/IEC 14496-12 for details on these “boxes”.

The HMMT box seems to be a non-standard (GoPro-specific) ISO/IEC 14496-12 box. Its data consists of one or more 32-bit integers. The first integer contains the number of available HiLight tags. All subsequent integers resemble an ordered list of HiLight tags. Each HiLight tag is represented as a millisecond value.