Given that the modern nation-state is a relatively new phenomenon, how did borders work in the past? Was there such a thing as “International” travel? Did people immigrate? Was there anything resembling customs or border control?
How did borders work in pre modern times?
I made a post two years ago on this subject in this same sub, which you can see here:
In short: "Borders" as we think of them today, and all of their accompanying geopolitical significance, did not exist in pre-modern times. This also means that there was no such thing as "immigration" (as opposed to simply "migration") and minimal - if any - "border control". The exception might be in heavily-contested frontier zones, where a more advanced state might attempt to implement some controls over the movement of people and goods. But with pre-modern technology, strict surveillance and control was simply impossible.