A restart results in the entire boot process described in section 3.2 of the FOTA User’s Guide being followed. So following a FOTA update, the bootloader is executed and it looks for a valid application. It should find the application that was just newly installed and execute it starting from the new main().
If you’re looking at the RSL10 registers that indicate the cause of the reset, I believe you’ll see CM3_SW_RESET_FLAG set. However the existence of this reset does not necessarily mean that the software reset was issued due to a FOTA update. That is application dependent on whether other events may cause the application to issue a reset.
If you want an application to be able to check for whether it’s a new version or not, I may suggest having the application store it’s revision in NVM. When the application starts it can check the version in NVM against it’s own version and if the values don’t match, execute the special code you want to run, and then update the value in NVM.