is siri avaliable on all iMac

Please click https://support.apple.com/en-in/guide/mac-help/mchl6b029310/mac to learn how to enable Siri.

Look in the System/Siri preference pane:

Siri capability came with macOS 10.12 "Sierra" in Sept 2016 (source). What macOS version do you have and what year is your iMac?