Inside a junOS device where is the image store path/.tgz , so that I
can copy from one to another ?(example in a vc - i can copy from
master to Line card)
You as the operator decide where the installation file goes. This is the file you'd reference in the
request system software command.
Once installed, Junos is no longer contained in a single image file. It is decompressed and the various installation packages that make up Junos are distributed across the filesystem. You'd have to use the original installation file.
Since you're in a VC, you might be able to use the automatic software upgrade feature. You'll have to have some form of access to the device to reboot it. Check the documentation for more details, note that it may not explicitly say EX3300, it should still work.
In general can anyone explain the bootprocess (note:not how to boot
from usb or tftp) on a Juniper device? what happens to the .tgz file
once its booted up? i dont see them in /altroot or /altconfig
Junos and Junos Evo are based on FreeBSD and Linux respectively (depends on platform/version) and uses a bootloader, there is no single image. The SRX345 and EX3300 run Junos (not Evo).