How to response back to UI upon REST API call within process execution

Hi, I’m stuck at sending response back to UI (mobile app) upon REST API call to backend.

My process is in execution state and one of service task has completed its work, now the job completed by this service task (java delegate) needs to be sent to UI.

I couldn’t do it, and then I replaced it to receiver task which will wait for an API call from mobile UI and return back the response once completed their job and then proceed further for execution of same process.

What is the right way to receive REST API request from UI when process is in execution state and response back to API call ?

Once job is completed by prepare & load UI workflow (java delegate), I need to send the workflow to UI for rendering, how can we achieve it?

Here is process diagram -