I think we're going to have wait longer to see what really comes next (Vol. 10 isn't due out till Feb 2019). It's still very interesting to see to what extent the anime and the manga are similar - and Yamazaki-sensei's comments at the end are (as ever) well worth reading.
Girls' Last Tour Vol 4
And so we reach the end of the available English volumes and also where the anime finished. There was even less in the manga about where the girls come from than in the anime. I hope the last 2 volumes have some answers, but that will have to wait till December! (and then February)