Copying multiple words (from different lines) and paste them at once

Use an uppercase register when yanking (copying):

"Ay

"A says to append to the a register, as opposed to "a which would replace the contents of the a register.

Once you've copied everything into the register, you can then paste it all at once with:

"ap

You could also define your own custom operator which would yank incrementally any text-object or text covered by a motion:

fu! Incremental_yank(type, ...) abort
    if a:type ==# 'char'
        norm! `[v`]y
    elseif a:type ==# 'line'
        norm! '[V']y
    elseif a:0
        norm! gvy
    endif

    call setreg('z', @".(a:type ==# 'char' ? ' ' : ''), 'a' . getregtype('"'))
    call setreg('"', @z, getregtype('z'))
endfu

nno <silent> zy  :<C-U>set opfunc=Incremental_yank<CR>g@
xno <silent> zy  :<C-U>call Incremental_yank(visualmode(), 1)<CR>
nno <silent> zyy :<C-U>set opfunc=Incremental_yank<Bar>exe 'norm! '.v:count1.'g@_'<CR>

nno <silent> zyc :<C-U>let [@", @z] = ['', '']<CR>p

In this example, 4 key bindings are installed:

• zy normal operator to incrementally yank a text-object or motion
• zyy similar operator which works on lines
• zy similar operator which works on visual selection
• zyc normal command to empty the registers " and z; useful before beginning to yank a sequence of texts

With it, you could:

• copy some words hitting zyiw on the first word, then repeat the operation with the dot command on the next ones
• copy some lines hitting zyy on the first one, then repeat the operation with the dot command on the next ones
• copy some visual selections, hitting zy on each of them

The current code clutters the z register, if you prefer using another register, like x for example, you could change the following lines:

call setreg('z', @", 'a' . getregtype('"'))                call setreg('x', @", 'a' . getregtype('"'))
             ^                                                          ^
call setreg('"', @z, getregtype('z'))                 →    call setreg('"', @x, getregtype('x'))
                  ^              ^                                           ^              ^
nno <silent> zyc :<C-U>let [@", @z] = ['', '']<CR>p        nno <silent> zyc :<C-U>let [@", @x] = ['', '']<CR>p
                                 ^                                                          ^

For more information, see:

:h g@
:h getregtype()
:h setreg()