Plugins and Apps can be found here:
Downloads - Invision Community
Help Guides can be found here:
In addition to reading through the forums you can learn more about the platform by reading the company blogs:
Invision Community Blog - Invision Community
I would also recommend using the search function to see if any questions you may have in the future have already been answered by the IPS staff or other members. Sometimes you might find a thread that answers your question, and sometimes you won't, but it tends to be worth the effort overall.
This suite is highly configurable, but alas not every single option has a tutorial on how to set it up. Many settings in one section of the ACP may impact other functionality so it's best not to make changes to the default settings without some thought as to the possible consequences. I would recommend setting up a test server to try out things and when you are confident that a new feature works for your environment you implement it on your live server.
Also note that some features like "Achievements" and reactions are setup for you on install. These features may or may not work for your community using the default settings so you may have to tweak them to work for you.
Often times admins think they know their community better than they actually do, as such, I would recommend working closely with your community to ensure the platform you develop works well for them.