To me, it feels like it’s organized like a PDF or other digital document. The rules glossary especially feels like an appendix that other sections of the text would hyperlink to, and indeed, in D&D Beyond it does work that way. Unfortunately, they didn’t include the paper format equivalent (page number references) in the printed book. As a result, the rules glossary ends up being harder to navigate in paper than it should be. It’s still a lot better than the 2014 version, but there’s a lot of room for improvement.
Probably more a result of a style guide than a common toolset. I don’t know how WotC does it for D&D, but at least for MtG they use a variety of artists, all of whom make the original pieces in traditional media. WotC recognizes that their artists can make more money selling those original pieces, so they don’t demand digital for their art commissions.