Aliens all eventually exit their ship

[Hobbes]

But are they choosing their target points randomly, or is there some sort of directive, to distribute themselves somehow?

The node points are chosen randomly

As far as the  maps themselves drawing them out in an unmodded game... in checking the farm(in MAPVIEW), according to their routing priorities, most route tiles were priority 0-2, though one 2nd floor tile in a barn before a window  was priority 8, which in part explains the fascination that aliens have with peering out at least one of the barn windows. 

Higher priority like 6 or 7 means they should patrol/guard that tile over 0 or 1.

There are routing map priories inside the ship as described here:
https://www.ufopaedia.org/index.php/Tactical_AI

Notice outside of the bigger ships it is 0 or 1. 
On the Large Scout (must see it in Mapview as not shown there) the priory of the square nearest the door is 6 as well as adjacent tiles inside the ship. The route priority of the route tile outside is 2 which means they probably are encouraged to move around a lot in the ship, but priority 2 should give them a higher chance of going outside more than 1 or zero.

Usually the nodes with the highest priorities (5+) are only used by specific alien ranks, otherwise all the aliens would eventually converge on these locations

[7Saturn]

So in essence, it's kind of statistical physics that they »poor out« of the ship over time, until the point where it's round 20. Thanks for the insight.

[SteamXCOM]

Hobbes from another thread gave more detail

The default mode for each alien unit is Patrol - each turn the alien will choose a node and move between them. Once the alien sees an enemy unit the AI will switch the unit's individual mode to either Combat/Sniper/Escape depending on its rank, aggression stat and RNG. It will then remain on this state until the enemy unit is killed or, in case it can't see the enemy unit anymore, it will remember its location for X turns with X being determined by its intelligence stat before reverting back to Patrol.

.............. As for the turn 20 timer, what it does is that after turn 20 all the alien units know the location of all your units. They will not Patrol anymore but they'll switch to either Combat/Sniper/Escape

Thanks Hobbes!!