A/B (Seamless) update device questions

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:

1. Here is a more detailed picture from Android Police

   
   



2. Also see A/B (Seamless) System Updates from Android developers.