how to know which file to open?

I'm trying to use editor for the first time. I'm just trying to combine a couple things. I've seen how to do it with opening one item, saving a copy, then adding in additional text(in my case, cloak of resistance +2 and a cloak of arachnida). However, the issue I can't seme to figure out is WHERE to find the right data file where I can copy the cloak info for this new file. How do you figure out WHICH file is the right one with all the different thigs that have been downloaded? I didn't see one that seemed like the natural choice that would just was just named like "basic items" or standard magic items or whatever.

TIA

In a new file, use the "New (Copy)" button to copy an existing item, and then you modify it from there. New (Copy) will open a list of everything that exists within that category, so you don't need to know how it's stored internally.