Offline mode on phones, smartphones and tablets is a mode that stops the transfer of data, signal (calls), text messages, etc. At the same time, functions that do not require signal transmission are available in this mode..
If we are talking about a smartphone, in simple terms, you get a handheld computer, which offline does not allow you to make calls, send or receive messages, use the Internet. All other features will be available..
The autonomous mode was created primarily for flying on an airplane, where the reception and transmission of signals can have an extremely negative effect on the on-board systems of the airliner. However, offline mode can also be used if, for example, you just want to be inaccessible for calls from friends, relatives, colleagues, or anyone else.
Interestingly, autonomous mode is most often called flight mode, although there is essentially no difference between them. These are just two different names, as you like, that one calls this mode. For example, in Samsung it is called offline mode:
On some other devices, this is exactly the flight mode:
But, again, for the user it does not matter.