SuccessFactors ODATA API feed from custom ABAP program

Dear Community,

I'm looking for a high-level and general guide or tutorial, explaining how to feed any SuccessFactors OData API (e.g. Onboarding 2.0 Service Operations) from an ABAP program. Let's take the ONB2 Service Operation /initiateOnboardingForUser (https://api.sap.com/api/OnboardingOBX/resource) as an example. This operation is to be called with three attributes - userID, atsApplicationId and startDate - to initiate the onboarding process for an existing Onboardee.

As you might see from my questions, I do not have any clear picture yet and would like to change that :)

1. How do I technically implement the data collection/processing?
Do I code that into function module in a function group or in a simple report?

2. How do I create a WebService from that data collection/processing function?
Is that similar to what is described in that blog: https://blogs.sap.com/2012/10/24/how-to-create-web-services-abap-based/

3. Do I have to create a consumer proxy, that needs to be configured in SOAMANAGER with a logical port that addresses the receiving endpoint directly?
Is that similar to what is described in that blog: https://blogs.sap.com/2012/10/24/creating-a-service-consumer-proxy/

4. Assuming I properly collected the required data, how do I phrase and execute the above mentioned Upsert?

5. I assume, I do not necessarily need a middleware, do I?

6. What about security? How is e.g. a token-based authentication handled and configured in such a scenario?

I'm happy for any supporting feedback ... is it a blog, a youtube tutorial or whatever ... it makes me happy and brings me forward! All my research so far did not give me an understandable starting point .... not sure I'm using the right search terms.

Appreciate your feedback and expertise!