Background Info: I have created a notebook (in the Wolfram Language using the Desktop environment) that generates a user interface. I have also created 15 other notebooks that generate different kinds of data for the interface, if the user calls for it. Currently the 15 "back end" notebooks are set up as APIFunctions and can be deployed so that a $post request from an HTML page or query via a URL will return the data. That was fine when my front end was an HTML page, but now I'm trying to switch it so that the user interface is written in Wolfram Language (Grid[], Pane[], Button[], etc.) instead of HTML.

Main Question: What functions do I need to use so that the front end UI (notebook? Instant API?) asks for and gets data from the back end (cloud object? Instant API?)?

Obviously, I'm a beginner in the Wolfram Language, so please be patient with my cluelessness, and answer in a way that will make sense to me as I climb the learning curve. : )

Thanks in advance,

Mark