I noticed the links now on the main Google or IG pages open on the same page- but the same type links on Gmail open up a new page. Any reason why?

And is there a reason the links are ordered they way they are, rather than, say, alphabetical order?

That's the way it has always been. The links for Images, Video, Maps, and News are now just on the top instead of above the search bar. And in Gmail, Docs, and Calendar they have always opened in a new window.