Developing Java application within Box Enterprise to upload Files

We currently have the Box.com enterprise license but this is the first time when we are trying to develop a Java application to upload some Files. The requirement is to upload Files by a batch application and also provide access to a third party company personnel outside of our company to download those Files. I would like to know the steps that are required to do this. Based on my reading of the API documentation I understood the following. Please validate if it looks good and also add anything that I'm missing.

(1) For an application to upload Files a service account needs to be created. And the service account will have its own content that it will manage. I have couple of questions regarding this step.

How to create a service account which will just have access to its content ?

How would we give access to a third party company personnel to download those files ?

Whats the difference between an App user and a service account ?

(2) The service account will use Outh 2.0 with JWT to do the authentication. Please let me know if there is any Java code sample using Box SDK.