Sometimes I see a window open with name scratch. I know it is some kind of temporary buffer/window. How can I crate one?:h scratch returns nothing.
What is a scratch window?
buffers;vim windows
It is probably a scratch buffer, which can be named. From the help:scratchContains text that can be discarded at any time.  It is kept        when closing the window, it must be deleted explicitly.        Settings: >      :setlocal buftype=nofile      :setlocal bufhidden=hide      :setlocal noswapfile  The buffer name can be used to identify the buffer, if you give it a meaningful name.This can be found in :help special-buffers.