JetPack installs to the desktop PC. JetPack can serve as a front end to the flash program. JetPack also serves as a handy way to install other packages on both the Jetson and the host. To flash and do nothing else you can use the command line driver package plus sample rootfs…JetPack arranges all that for you if you use JetPack to do the work.
When JetPack runs it is on 64-bit since the PC is 64-bit (x86_64 architecture). The operating system it installs onto the Jetson is 32-bit ARM architecture (armhf)…this is normal, the JTK1 is 32-bit. JetPack itself is not able to run on ARM architecture. The Ubuntu operating system put on the Jetson is called L4T (Linux for Tegra)…this is standard Ubuntu plus direct hardware access files.