Should i create a web-service to provide the app versions based on the
platforms?
=> As such Android provides notification whenever new version is available, but as you mentioned its your business requirement, so i assume you want to implement/include some new features in new versions only.
so check version and implement such, you must have to have web service for the same.
What data do i need in the web service to keep it generic? Android
uses version code where as iOS uses major and minor version updates.
=> You can fetch version number from the current app. Check: Get application version programmatically in android
The app is login based. Should i check for the updates after the user
logs in or before?
=> There are 2 cases:
You can call web service and check whenever app is launched. (May be in splash screen or something else)
If you do include this functionality (version checking) inside Login, then you must have to check everything whether the user is already logged in or not. So i think this is not a good case.