It’s mentioned in the symbol library format document at the bottom of the page. While “inherited”, “extends”, etc., sounds big, it currently only provides little more functionality compared to the old file format, IMHO (apart from being better suited for git). It is implemented differently, but not fully. What’s new is that fields can be overridden in a derived symbol. That and the potentially higher resolution of coordinates would make it potentially incompatible with the old file format if these features were used… (And I’m starting to suspect that the hard push for the new incompatible half-baked file formats could possibly have to do with KiCad file importers of somewhat competing products…)
If you open an old library with aliases and then save it in the new format, you can see how it’s used in the file.