I also can't figure it out. I still have some window that appears on the side and I can't figure out how to completely get rid of it (from my experiments with multitasking while I was reading the documentation, or at least some website describing it). I don't know if that's intended but I have no idea how to use it, so don't.
I had an easier time learning Xmonad back in the day, which might say something about Apple's UI design these days.
Yes - I have the same experience.
I use the iPad Pro and Juno connect as my main development tools, together with a VNC connection to an AWS instance for tasks that need a desktop.
I tried to use split screen initially but found little value in it and now ignore.
Just an anecdote. I am a very technical user and I still got confused by the multitasking / slide-over thing. So I just ignore it.