1) They're submitted by third-parties. There is no official "vetting" process, if that's what you mean. But problems/questions concerning a particular plugin are typically brought up in that plugin's own thread. Which brings us to number two ...
2) Each plugin has its own thread in the Sigil
Plugin Subforum. That thread can be found by browsing the Plugin Subforum or by following the link in that plugin's entry in the
Plugin Index. The kindlegen plugin's own thread can be found
here. That's where you'll find a description of its purpose, and discussions about its usage and/or potential issues.