Try this video for some basics. It does say that you use Cantina energy for mod challenges & battles, but that's because that's how mods started. It took a while before they got their own, separate energy source. I'm starting you 7 minutes in:
Hopefully that will show you some basics about how the interface works.
Then you can read this article, which will also help:
https://gaming-fans.com/star-wars-goh/swgoh-101/swgoh-101-comprehensive-mod-guide/
After that, hopefully you have your current questions answered. If not, at least you'll have a better idea of how to formulate the questions you have left.
