Quora has answers on on the tech stack in this thread: https://www.quora.com/What-was-the-technology-stack-driving-...
Reddit has answers on the rendering engine here: https://www.reddit.com/r/gamedev/comments/d3cd2v/anyone_know...
I contributed replies to both, but there's further detail from others.
There are piles of Ultima Online postmortem materials on my website, https://www.raphkoster.com -- and those and more are collected in my book "Postmortems."