Hi Adraline !
Nowadays people often work alone on Age modeling, but there are still collaborative projects from time to time. Besides, we don't have a lot of Age builders, so help will probably be welcome !
We mostly use Uru's game engine (called Plasma). It's quite old, but is still being improved by a few fans. It's certainly not as good as Unreal or Unity, but supports multiplayer quite well. Right now, multiplayer for fan-made Ages is only available on fan-made servers like Gehn or Deep Island, but in the future some of the Ages will probably be included in Cyan's Uru Live servers.
The engine does not have an editor like Unreal/Unity though: setting up the scene is done in either 3dsMax (using Cyan's official plugin), or in Blender (using fan-made plugins). Overall it make building much harder, but that all we got, unfortunately.
There are a few projects on Unreal and Unity though. I know Yali is working on a big project using Unreal. I'm working on converting existing Ages to work on Unity and in VR (although currently it's more of a personal hobby). And there are a few people who are interested in newer engines, although not a lot actually use them.
I'd love to build an entire age but I really don't have the time or programming skills yet to conquer it all
Well, you can focus on building a smaller Age (the size of Eder Tsogal for instance) if you prefer. And a few of us can handle the programming part.
Libraries of textures, hmmm... Well, the way the game is coded doesn't make reusing Cyan's textures easy, so it will probably be easier to find textures elsewhere (like at textures.com or at one of the websites in this post
By the way, Myst and Uru are also the reason I got into computers myself
but on the programming side (while still doing some modeling in my free time). I'm preparing an engineer degree in computing, and working part of my time as a developer. Don't know what I'd be doing today if I didn't play Myst as a kid
Anyway. Keep poking around, there are plenty of fan-made Ages to explore, and a lot of resources about Age building in the Wiki. Some of it might be quite confusing at first, so don't hesitate to ask questions in the forum