Both can be either scratch built (where you do it yourself) or assembled from kits. Have a look at the Walthers website (www.walthers.com
) to get an idea for what is available. Their "Magic of Model Railroading" section also has some good photos.
Also, try looking at Model Railroad Hobbyist magazine. It is an online only publication, free to subscribe, and the back issues are fully indexed for articles.