Hi Krishna,
this blog posts shows a simple example for creating a new launchpad GUIBB from scratch. At the end, you can start from this page different kind of applications, for example a SAPUI5-App. Of course, you can just create simple LinkToURL/LinkToAction controls in your WD4A-Views, but this approach is especially useful, if the possible navigations depends on the users authorization and if you want to use pure FPM-technology(the Launchpad is a part of the FPM/WD4A Framework). This official documentation offers some basic information about that stuff.
If you want to start an application from your SAPUI5 app, you can just embed URLs into your code as simple Link-Controls. But when you need to generate possible navigation-targets dynamically into your site, you can just write a service, which delivers all navigation-targets based on the users server-side authorization (or other criteria).
Data exchange can be done in many ways. Shared Memory, Database Table, Wiring (FPM), IF_FPM_SHARED_DATA(FPM), Simple Classes(FPM) etc. The problem is, that you always have to use a service (Gateway, SICF or whatever), if you want to share data between your backend and your SAPUI5-Application (which is mostly stateless and purely client-side-implemented).
Hope that will help
Regards Michael