My Command-line Inform post also goes into detail about Inform projects’ file structure. The most crucial parts are that the story file goes in project-name.inform/Source/story.ni and included extensions must be available as either project-name.materials/Extensions/Author Name/Extension Name.i7x or External Dir/Extensions/Author Name/Extension Name.i7x. My post explains what the External Dir is; where it is exactly depends on what OS are you using. What OS are you using?
The websites you see that offer source are what you get by using Release along with a website as described in Writing in Inform chapter 25: Releasing. You may not be able to actually build these projects, as these websites don’t include Extensions the project may have used, and they may not all be public and findable.
Also note that anything older than May 2014 was written for Inform 7 6G60 or earlier and is unlikely to compile without modification in current Inform 7, 6M62. And, of course, there are still some people using 6G60 now; something being recent is no guarantee of 6M62 compatibility.