In Debian if I halt the PC correct way, all the external mounted devices (Flash-memory, external HDDs, cameras, etc...) umount automatically so I don't need to umount them manually to unplug after halt? Right?

If the computer isn't running (halted), how would you unmount the device? Of course, when the computer is down, the devices are safe to be disconnected.

Right.

Part of the shutdown process is to unmount all mounted filesystems.

If you disabled your "splash screen", you'd see the kernel messages going past as your machine is shutting down [ You'd also get to see the startup messages, these can be informative and useful ]. Somewhere near the end of the shutdown process comes "Unmounting remaining filesystems", very shortly after that, it halts.

Once the machine has powered off, it is safe to remove any external drives.
If you want to remove a mounted drive without powering down your machine, you must unmount it manually (ckicky-ckicky in the GUI, or the umount command in a terminal).

Thanks!

But what about the scenario when the system is going into the suspend mode?

Good question, but:

1] If you have a new question, please start a new thread, with an appropriate title.

2] When you start your new thread please tell us what "the system" is, exactly: You might know, but we cannot read your mind, and you have not told us. Perhaps you are running windows-xx?.

3] If you do not tell us your hardware, or "system" you are running it is very difficult for us to help you further.