Hi Chad,
The JS library is using localStorage to store "sessions id", "profile id" and "anonymous id". Username/emails/passwords are not stored. Only the information necessary to restore a previous session. As for implementation details about localStorage, it all depends on the app framework/browser you are using.
There is no native code in our JS library, correct. We do have native versions of the library (C++ and Objective-C). But if you want a web app, those are not useful to you.
We have an NPM module for it and a minified .js file for webpages. There are many frameworks out there, so there might be minor code change to be done depending on which framework you are using.
If you are using our JS library, even inside a framework or on any device (You mentioned Android), the platform used by brainCloud is always going to be "WEB".
Hope this helps. Did I understand your question(s) correctly?
David