We have video tutorials which explain most of the things you mentioned, combined with the online documentation you should have everything you need to know
opsive.com
As for API docs, the code is fully documented so going through the classes and public functions using your IDE, using the autocomplete and reading the comments and description of the relevant classes/function should give you everything you need to know. We don't plan to copy paste all that information in the online documentation. All the API that is "Essential" are given as "code examples" in the documentation (for example the sections Item, Attribute, Item View Slot Container, etc...)
We continuously update the documentation with more information when users say something in particular is not clear. So if you have a section in mind that needs to be improved let us know.
If you read the documentation carefully you would have seen that the API question you asked is actually part of the code examples in the documentation page:
The base class “Item View Slots Container Base” is used by the Inventory Grid, Item Hotbar, Item Slot Collection View and more. Some options in . . .
opsive.com
Code:
// Get the item in a slot.
var itemViewSlot = m_ItemViewSlotContainer.GetItemViewSlot(slotIndex);
var itemView = m_ItemViewSlotContainer.GetItemView(slotIndex);
var itemInfo = m_ItemViewSlotContainer.GetItemAt(slotIndex);
But even if it was not your IDE should have been able to autocomplete this if you had tried different keywords ("Item", "Get", "Slot", "View", etc...).
If you aren't using an IDE, like Visual Studio (Not Code) or Rider, I highly recommend you do it'll save you hours of work each week.
As for your last question Currently there is no easy way to drag and drop items in screen space. Some people have added a giant Item View Slot to cover the screen and used custom code to listen to the drop event.
That feature is in my TODO list in mid-priority so I'll try to add it either in the next update or the one after.
I hope that helps