Hi Darshan,
BY connection do you mean getting data from ABAP systems and populate it in UI ?
If yes, then you can introduce Gateway system in middle of ABAP and frontend. You have to connect abap and gateway system with trusted RFC and create one gateway service which will get the data from backend system,
Once you have the service, you can create a UI5 application and consume this service to populate data in the UI.
Let me know if my response is not clear.
Regards,
Tejas