Congrats! That is great news to hear!
CSV would need to be the format you use as we do not support importing products via XML. I would recommend supplying them with the CSV template that you’ve found in the Help Centre. In addition, they can view our CSV help document which describes each column and how they are used. If the supplier isn’t able to provide you with a CSV that aligns with our Columns, then you’ll need to manually update the CSV so it can be imported to Shopify.
In terms of the product tree, I think it would be best to ask them to clarify this for you. It’s better to ask than to assume so ensure you’re both on the same page about what the next steps are.