Not sure if this is what they did, but you can just put all the things you need together sequentially into a single file and rely on the filesystem to allocate

Not sure if this is what they did, but you can just put all the things you need together sequentially into a single file and rely on the filesystem to allocate contiguous blocks where possible (using the appropriate size hints to help). It's trivial unpack at loading time without any performance impact. A filesystem is by itself just one big "file" acting like a file archive.