To unlock the "SAFE HARBOR" trophy in Detroit: Become Human, you must make sure Kara and Alice reach Canada by going through border control. Crossing the river will not count.
Here is one example branch of the flowchart "BATTLE FOR DETROIT - KARA LEAVING DETROIT" to get Kara, Alice and Luther across the border, assuming you've managed to get Kara and Alice this far and Luther hasn't been killed. If you get Luther out as well you can get this trophy at the same time:
Bear in mind that this is one of perhaps many possible routes to getting Kara and Alice through border security. I will update with other options as they are discovered.
If you get Kara and Alice (at least) through official border security by the game's closing credits, you will get the trophy when you reach the flowchart screen.