How can I upload a feed and understand API in Shopify?

Product Development & Sourcing
1

Hi there,

Help me please to understand this topic better, because I’m pretty new in Shopify, too!

  1. I have to upload a feed from my new seller, it’s that possible? It implies any extra costs for me?

  2. What is more exactly that API Shopify and this app support Ella Theme?

  3. Also, please, recommend me a new theme suitable for DIY products, products that have many variations, sizes, colours (product specifications). I already bought the Ella theme, can I use something from there?

Thank you,