Xcode User Interface Builder: Do correctly configured XIB examples exist?

Could you be more specific ?

What xib are you looking for ? What is the problem you don't know how to solve ?
What do you mean by "configuration" ?

If you google for "XCode xib examples", you will find a lot of tutorials.
Just an example:
https ://medium. com/@umairhassanbaig/ios-swift-creating-a-custom-view-with-xib-ace878cd41c5

As I said above already, there is no shortage of google searches ("XCode xib examples") and tutorials that describe the theory of MacOS user interface design. The limitations of most tutorials however can be described as "how to draw an owl".

What I am looking for is best practise for realistic user interfaces of a realistic level of complexity - and obviously the best of best practises will come from Apple themselves.

Does such a thing exist?

I don't know of such material. It looks like Apple is no more focusing on this and counts on the forum(s) (this one and other) for "community education".

So the best is to ask questions on the forum when you have design doubts or want to be challenged on one of your xib designs (but forum does not accepts images, which make things even harder to explain, unless you can provide a link on imgur or other).