What's The Deal With Spirit Weapons?

The title is relatively self-explanatory, but let me lay out some more specific questions:

What is the In-Verse, canon explanation for spirit weapons?

Where do they come from, how are they obtained?

How did Orochimaru know about them?

How did Itachi happen to get a hold of the unstoppable spear and unbreakable shield In-Verse?

Why didn't anyone else attempt to obtain or utilize these weapons?

This feels like a major plot point that was dropped in favor of focusing on tree hugging aliens, does anyone have any explanation?