Basicly the same as the PKG file format before but with one change. The pkg is not a own container which is additional encrypted anymore. It is a renamed non encrpyted .zip file which holds encrypted files inside.
Make your own PKGS files
Simple use HENkaku to enable Homebrew and then some file serving tool. Either VitaShell or a FTP Client will do the work. Copy the whole content of the Title_ID folder from your desired app to a new location. Then delete the folder package, pack the other files and folders into a .zip and rename it to pkgs.
Now you can use your pkgs and install it like every other one too.
Attemption ! This will not remove any DRM !