It depends on the users! Some prefer to write over messages, some prefer to write over thread, some prefer to write over email, discord, or other off-site mediums. There are group roleplays, and they have their own search and posting sections in the forums.
I think that most people do some amount of plot and character design ahead of time but it really depends on the writers.
(Delete's entire post because Officer @Virginia Greene was faster than me at the draw. I''d be a dead man going against her 0_0)
Indeed. This site is quite the melting pot, so people vary. Your best bet is to scout out the request threads and maybe scan over some on going roleplays to see how things play out.
I think it would depend on the person. I like discord or open to other message site like that, gives it a continuous feel.
Good luck. I am new to this site as well.