Are there any specific changes under the hood in SDL Web 8.5 vs. SDL Tridion Sites 9 that are note worthy from an architectural and installation point of view.
At SDL Connect in October last year the roadmap and capabilities for SDL Tridion Sites 9 were presented, you can view that presentation here https://www.sdl.com/video/roadmap-and-capabilities-for-sdl-tridion-sites/125664/
Before that there was a similar webinar, from which you can find the recording here https://www.sdl.com/olp/digital-experience/web/roadmap-and-strategy-update-for-sdl-web-tridion.html
But at this point the final details of the upcoming Sites 9 release are not yet available. In general I'm not expecting significant changes from an architectural and installation point of view. I'm expecting it to be continuing on the same route as Web 8 and Web 8.5. With support for rolling upgrades being the biggest change in there.