tar is a “tarball” file. This means that lots of files can be stored in one “tarball”. That’s similar to what zip does but, unlike zip, tar does not compress. Therefore you will often see archives for Linux devices with a tar.gz extension. This means that the file has been “tarballed” and then “zipped” with gzip (a Linux zip program). So if you open a tar.gz file with 7-Zip you will see a tar file inside and if you then click that with 7-Zip you will see the contents (7-Zip supports “nested” archive files). The JEX file is just a tar file (not tar.gz) so clicking on it with 7-Zip just shows the contents.
Thanks for the excellent explanation. Caused me to look at the 7z help file. Found you can do the same with 7z i.e archive copy mode (no compression) using the -m (Set compression Method) switch -mx=0.