[HELP]How the application image communicate with factory image?

Hello, everyone

I'm now confusing with the issue about the remote system upgrade. I have read some relative posts in this forum and basically understood the working flow. But the following questions still make me confusing:

1. Does the software of factory image still run after reconfiguration to application image?

2. Which image(i.e. factory or application) should have a procedure to receive the upgrade user image and to store it in EPCS device?

3. If I enable the watchdog timer, how can I reset the timer in application image?

4. Since the remote update core is in factory image, how can I access the register of this core in application image?

Thank you for your help!

wei