Provide opening = enough space for the windows. Or they may charge you removal of the grills or old windows.

Plaster = After brick work, you need to put cement to make it like normal wall, this is call plastering.

Usually window frame are install after brick work, because they need to mount the windows frame holder / bracket to the brick, then the wet work contractor will do plaster. This look nicer.  If you have already plaster they may need to hack and put the bracket and plaster back. So this may not turn out to be nice  and may charge you extra.