Need help in fetching app version of an another installed app on iPhone

Guessing that you mean some arbitrary other app not developed by your team: there is no API that can give you that information. Unlike on Android, the iOS platform has no APIs to discover what other apps are even installed [*], much less tell you anything about them. That’s example of how the concept of user privacy can differ between the platforms.

[*] There’s actually a small crack in the armor with an API called canOpenURL (link) but it’s intended for a limited sort of cross-app integration, not for open-ended discovery of other apps, and tells you nothing about version numbers.

What Scott said plus…

If you’re app is deployed to a managed environment, it’s possible for the MDM system to know what apps are installed [1] and you could potentially have your app ask the MDM system about that.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

[1] Although there are limits on that too.