To what extend did Istari know Middle-earth?

[JeyEn]

Hello! As I am reading Unfinished tales, I found this passage:

Wizard is a translation of Quenya istar (Sindarin ithron): one of the members of an "order" (as they call it), claiming to possess, and exhibiting, eminent knowledge of the history and nature the World.

So in my earlier post, I asked about Gandalf's knowledge about Bilbo's ring. This passage from Unfinished tales says they had a vast knowledge of the World. So my questions:

1. In what sense does Tolkien mean that?

2. How much did Istari know of Middle-earth? If they had knowledge of the World, they should have known Middle-earth, its people and history.

3. If they knew Middle-earth's history, they could have known much about Rings of Power, no? Therefore, Gandalf could have known much about it -> much earlier conclusion about the Ring. But I suspect that this part of my questions are stupid, since they as Maiar probably didn't visit Middle-earth, and if they did, they couldn't appear anywhere they wanted.

4. What "history and nature" is meant?

As Maiar, they probably didn't know much about Middle-earth, but if they did, Gandalf could have known about Rings of Power much more - which is a stretch, since he obviously knew very little before his further investigation.

Thanks for answers, I found this forum very much useful!!