Hi Veer,
Thank you so much so much so much. It was like a nightmare for me. As you said, when I opened up new clean project with unity and only admob, to my surprise, it worked. Then I found what I was missing, Re-opened my project, deleted the folders and files created by admob...(specifically the plugins folder...which was the issue...project was for android and there were some files modified before to suit my need) re-imported admob plugin...built...added the line pod 'Google/SignIn' to the pod file...open terminal...go to the folder path and type pod install...open xcworkspace file with xcode...and only added $(inherited) to the Unity-iPhone project (not the pods project...previously I added here too)...and to my surprise...with your help...build succeeded. I have described like this workflow so that if somebody stumbles upon here with the issue...s/he may find it helpful.
Thank you again.