where can you find the well-made adventure? is there a download page? Are the adventures made as challenging and beautiful as the original game? or is it all that much easier. how difficult is it to make such an adventure yourself.

If you are talking about Game Master campaigns, I think they are pretty easy to develop using GM mode. There are lots of extensions/maps/scenes/etc. available in the Steam Workshop.

Apparently you can develop your own storyline if that's what you are talking about. It requires using the Editor. I've tinkered with it, and find it rather intimidating, much more difficult than the GM tools. I'm not aware of any being published, but if there are, they should be available in the Steam Workshop.