Hi Laman
I am not sure if you are asking about a magic way of hooking into the process engine to get data loaded from somewhere else based on references. That is not how this works. By using references in your process instances, you avoid having the processes instances hold complex data and instead you lave that to the applications that own the data.
The process instances only save process variables that contain references to data - fx ID’s, keys etc. When you need this data you ask the applications for it and this is something you will have to implement yourself - either using Java classes or external tasks. Fx by calling a REST API’s or web services.
You may also consider the API’s your process is calling. Fx instead of returning a big complex data objects to your process, you could let your API’s expose simple business domain operations asking about the state of an entity. This way you can save the answer as a process variable and use the variable for process control decisions etc.
Hope that helps.
BR
Michael