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.
The current functionality is only the first step. This will be implemented slowly over time as it involves major changes how symbols are handled internally and the symbol editor interface will have to be updated as well.
If Pcbnew and Eeschema were the tools (let’s say, hammer and screwdriver, respectively), then it looks like for V6 there will be nicely placed nails (PCBs), and metric screws hammered into imperial nuts, all fixed with hot glue (schematics with snapping)…