How do I move around and otherwise rearrange splits in vim?

I'm sure the help file covers it, but to move a split to cover the top of the screen, you can do CTRL-W K (i.e., Ctrl+W Shift+K), with H = left, J = bottom, K = top, L = right (following usual vi/vim conventions).

Note that the letters are uppercase. CTRL-W k (i.e., Ctrl+W K, without Shift) will instead move up between splits, etc.

I suggest you read through the help file for vim windows. The specific sections of that file that apply to your question:

:help opening-window
:help window-moving

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Edit: I can only assume this is periodically getting downvotes because I haven't provided specific answers here. The problem is that the original poster asked several questions, and they were broad. The best way to answer all those questions at once is to refer to the Vim documentation, which should always be available wherever Vim is installed.

Also I want to note that I didn't just tell the OP to "RTFM," I provided specific sections of the manual to read, which can be an acceptable answer.

You can do:

:to sp

or

:top split

to do a horizontal window that spans the top of the screen.