As you can see, vim pops up a window with definition of the function completion on the top (I don't know what its name is...), can I move it to the bottom?

As you can see, vim pops up a window with definition of the function completion on the top (I don't know what its name is...), can I move it to the bottom?

Put the following line in your vimrc. (Relevant help page)
set splitbelow
This causes all horizontal window splits to be put on the bottom.
Add below script to your vimrc file
"Preview window on the bottom of vim
function! PreviewDown()
if !&previewwindow
silent! wincmd P
endif
if &previewwindow
silent! wincmd J
silent! wincmd p
endif
endf
autocmd BufWinEnter * call PreviewDown()