My suggestion is to work it out for yourself. After a couple of attacks you will get a fair idea of where the game drops the foes and adjust your defences accordingly.
When things spawn I think they are dropped from above. I have seen them drop a couple of times.
Sometimes the invaders send an advance party with reinforcements approaching from the perimeter.
Some of the more organised foes spread out using ranged weapons and grenades utilising cover to take pot shots
At a few of my settlements there are piles of green goo that never go away.
Some of the settlers have plasma guns and the goo marks a spawn point.