XML and affiliate data linking. basic understanding

XML and affiliate data linking. Basic understanding
Goal – We would like to setup affiliate link data to some of the data from an affiliate account.
We do not want to do it ourselves, but would like to understand the process, so we dont make any stupid mistakes.
We will employ a developer to do it for us, but we must be able to simply add the individual listings ourselves.

This is the process as I understand it

this is what we understand
the XML file is the data source
we need a css style sheet to format the data
1. copy data from source
1a. XML file from another URL and store it on my hosting site.
1b. using a tool similar to this https://www.wpallimport.com/
1c. this way the link is not live and the reference is on our hosting site.

This is what we would like to know
How do we only copy what we need and not the whole database?
how do we create a reference in the post to copy the specific link data?

any input would be appreciated

thx
Darren