Multiple API integration

Hey guys, I need some advice on the following task.

I need to make an integration between 3 applications.

The first application (Python) will provide some data to a second application (Rails) which needs to send this data to a client application (any language). The client would process the data and return the result to the Rails application.

Imagining that this data is building addresses and we have more than 20,000 of them. What would be the best way to communicate with those applications? Obs: The data needs to be encrypted and I need to say to the client the specification on how we are going to do that.

Sending API request with the data as JSON in the body of the request in batches would be easier than sending a CSV/JSON file in a request.
We have been having problems with CSV files coming from other clients which do not format it very well.

I need to secure data integrity and security in all processes.

Do we have some courses here that could help with that?

Any advice?

Thanks so much for your time and attention.