Great questions! Please see answers inline.
Is this "bundle" unique to an app?
--> Yes, "bundle ID" is a unique identifier for a single app within that platform's "app store ecosystem" (ecosystem = Android:Google Play Store, iOS:Apple app store)
For example, the "bundle ID" (aka 'Android Package name') for the Android NYTimes app on the Google play store is 'com.nytimes.android' (listed in the URL). No other apps on the Google play store is permitted to use the bundle ID 'com.nytimes.android'. It is specific to the NYTimes Android app only.
On the iOS side, the "bundle ID" for the iOS NYTimes app on the Apple app store is 'id284862083' (listed in the URL). No other apps in the Apple app store is permitted to use the bundle ID 'id284862083'. It is specific to the NYTimes iOS app only.
Can a single app have multiple "bundle"?
--> No, a single app will have a 1-to-1 relationship with "bundle"
Can a single "bundle" might mean multiple apps?
--> No, I think you may be interpreting the term "bundle" literally. "Bundle" is also known as an "Android package" (this is set when you are developing your app). It uniquely identifies a single app. See this great answer on Bundle ID for more info.
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