How can I copy some content from one site to another site? I have two sites i'm working on (one in D8 8.4.5, the other in 8.5.0).
I'd like to "copy" some elements from one to the other.
For example, some nodes or some menus (with their links, of course) or some users.

How do I do that?  

With configuration sync and database tables export/import?
You'll need to use Drupal Core's migration module with Drupal 8 migration (source).


  This is an Migrate source to import Data from a Drupal 8 site.


If you're not familiar with creating custom migrations, you'll need to read up on that. Also read the README file.