web service to process data subject request

I am new to coding in c# and have a project to realise in VS 2019. like the new GDPR law say any person has the rights to know what personal data has been stored concerning him. I have to develop a restful service which would be called within an application. This service would have to search for all data regarding this person in other various applications.Which means any one using this service have to implement the interfaces of my webservice. I have the following questions.

How can i realise this since various applications have different ways of storing personal data. Meaning i cnt have a fixed model for the web service

How can i gather all information about this person i.e data stored in databases, pdfs, pics , videos of this person stored as well?

Thanks for your time in reading and any little input will be helpful for me to come through.