Q: Is iOS working in Stand by situation ?

Hi,

If you are  referring to Lower Power Mode ..

Check battery usage

Go to Settings > Battery.

To see how long each app has been open and onscreen, and running in the background, tap . Under each app, you might see these usage types:

• Background Activity means that your battery is used while apps run in the background.
• Audio means that apps play audio while running in the background.
• No Cell Coverage and Low Signal means that your device is searching for a signal or being used with a low signal.
• Backup & Restore indicates that your device has been backing up to iCloud or restoring from an iCloud backup.
• Location and Background Location means that your battery was used when tracking your device.

Above information available here >  About the battery usage on your iPhone, iPad, and iPod touch

Of course.  iOS is the operating system, so even the telephone function is under its control and that remains active in standby.  If you have push email on, or allow notifications to run in the background, those will continue to listen for incoming activity.

When plugged in and on standby, the wifi radio also remains powered on and iCloud backups will run automatically, once every 24hrs.

So yes, in standby mode the operating system is still running and doing things, and if you've enabled them, some background process (e.g push email) will also continue to run.

I'm not sure I understand what you mean?  iOS is a UNIX based operating system. As long as the system is powered on (battery or plugged in) the kernel is loaded into RAM and active. One thing that the operating system and various system daemons that are loaded during boot up are always doing is monitoring RAM indices, file system, I/O, system calls (e.g. a location-based alert or notification activates and the notifications system makes a call to iOS's location services sub-system) and other basic functions.  So as long as your iDevice is turned on there are active processes running on it.  But those system and background processes use very, very little power. The maximum suggested standby time in the device specifications includes those basic system services and functions running. The only way to stop those services and functions would be to power off the device.