I'll do my best to explain some of these questions.

----Ski-lodge: typically, a CBer creates a thread under an alias. They will ask for other CBers and sometimes AEs or captchas to post forms. People will join as themselves. From what I've seen, most ski-lodge creators have a definite plan, but may change a little based upon how interested the members are. Rarely, ski-lodge creators are way more rigid, and they will write out almost everything in advance, kind of like a form letter where names and pronouns could be filled in. Or, the creator will get the character forms and completely wing it. It depends upon the CBer and their writing style. 

Now, player input usually doesn't affect the storyline much; most creators will adjust a little if the players don't seem interested, or if the story is moving too quickly, but usually, the players can't really affect anything, because even the murderer doesn't know who they are. 

----RP games: Depending upon the theme of the role-play, CBers can join as either themselves or a single-use OC. Usually, CBers can choose whichever they want, but if only one option is allowed, the creator usually say somewhere on the original post.

Hope this helps! 

Basically, a Ski Lodge is (like you said), a solo-write mystery thing where someone starts it under an alias and people join as themselves and fill out forms for themselves and any AEs and/or CAPTCHAs that they wish to bring. Usually the writing is posted in days or parts of days (ex. Day 2 Part 3). Usually, it ends with the murderer having killed everyone and revealing themselves, but it doesn't have to work that way if you don't want it to. For me, who is a certified expert in planning out and starting Ski Lodges but never finishing them, I can say that I plan each day out roughly (ex. what happens, who dies, etc) and work out the rest as I write it, but it can be different for everyone. As for reader input, sometimes readers will write little diary entries about the day after it's been written or something along those lines.

For RPs, usually people use a charrie (character) that they make up for the RP, though some people do reuse charries if they don't want to keep making them up. It's really up to the rules that the creator sets up in the beginning. 

I hope this helps! Feel free to @ me if you have any more questions about anything :D 

Thanks! Both of these were very helpful. One more thing: Kyngdom! I know this is a complicated thing that possible deserves its own thread, but basically I've read many of the things summarizing what's happened, explaining the magic, etc. However, how do I navigate to the separate part of CB for Kyngdom? Also, what do you do once you are there? How much interlocking of stories is there? (as opposed to just different stories vaguely close in the same world). Sorry I have so many questions!