I found the following:
GET /process-definition/{id}/xml
provides the BPMN diagram (XML Data) (REST API | camunda BPM docs)GET /process-instance/{id}/activity-instances
provides the state of a process instance in a tree structure (REST API | camunda BPM docs)
I can’t seem to find a reference for how to apply the activity instance data to the BPMN diagram even though i can retrieve the raw data from the camunda end point.