A/B (Seamless) update device questions The internal storage of each Android device is separated into partitions such as system and boot. Most devices have only a single copy of each partition, but devices that support seamless updates have two copies of certain partitions—slot A and slot B, hence A/B—so that one copy of the partition lies dormant while the other is in active use. Because each partition is a copy, they can be used interchangeably to boot into Android. This also means that you can update one partition in the background and then swap to it when you reboot—thus your update is seamless!

I need to know more about this for a stock unrooted device:


After OTA system update, does the device install OS to both partitions? Or  install to only one partition. 
whether the device continues to same(current) android version until reboot manually?(after installing version update). Or whether it updates current to latest version overnight.
When normal reboot(not after update), how can a user identify whether device is booted to which slot (A/B)? Else, will the device continues to current slot on normal reboot(for example if it is A). 
If any one slot got corrupted, Is it possible to use other slot only.
After OTA system update, does the device install OS to both partitions? Or install to only one partition.

and

whether the device continues to same(current) android version until reboot manually?(after installing version update). Or whether it updates current to latest version overnight.

It installs only to the inactive partition, which becomes active on reboot after update. Figure explains clearly


When normal reboot(not after update), how can a user identify whether device is booted to which slot (A/B)? Else, will the device continues to current slot on normal reboot(for example if it is A)

On a normal reboot, it always reboots into the currently active partition. You can verify by running this command pre and post normal reboot
fastboot getvar all | grep “current-slot” (On Linux)
On Windows, fastboot getvar all and search for current-slot
Edit : You can see current slot using apps like Dev check

If any one slot got corrupted, Is it possible to use other slot only.

If you mean by corruption , the update has failed,

....the device will roll back to the working build on the other slot....Your data is kept perfectly intact, even if the update is borked, as there is only one partition (userdata) which houses your data.

From How A/B Partitions and Seamless Updates Affect Custom Development on XDA and for more technical details see A/B (Seamless) System Updates
Edit: Here is a  more detailed picture from Android Police