Trying to Understand the nature of Amazon Q Applications vs Amazon Q Across "The Enterprise"

Based on the documentation I have read so far, it appears to me that Amazon Q applications generate Reponses based on the Q Foundational Model + documents retrieved via a specific retriever (and data source) and its configured metadata attributes etc.

Then, in order to see responses for this "Q Customization / App" you need to build a specific web experience with the APIs, ie these API calls take a specific applicationId

For example (from the Q API Reference) POST /applications/applicationId/conversations?sync&userGroups=userGroups&userId=userId

Is it true that this application is completely "isolated" from Q responses you see across AWS Accounts in a given Organization?

Is this correct OR do all "applications" contribute to the Organizations overall responses even in the "absence of an applicationId"

Please help me understand how it works.

Thanks in advance!