For the first 50 turns they are scripted. Now this varies by nation. For the smaller ones, its basically a ring around your starting region(s). Others roughly reflect historical interests. Rome and the Antiochids have 'tracks', so basically if Rome pushes into N Italy/Gaul first you get the Gallic track, if you go across the Adriatic, you get the Greek track.
There was a little bit of early game balancing here, you'll see that some weaker factions have relatively easier access to their first cluster of objectives whereas one of the major powers may have to reach a bit further (that is why Rome doesn't get central Italy - but you have other scripted incentives there).
At T50, they become 'dynamic'. You'll find they become a bit like a trail of crumbs leading from where you are currently making gains or at war. They also get removed, for eg in my current game I had one in Massilia, a few turns later I made an alliance with them and it was removed and allocated somewhere else.
If you shift direction, those will start to be removed and replaced. Now as you can get into some pretty remote wars via alliances, you may find some of these are a trifle hard to achieve. Don't worry peace out, and over time it'll be removed and replaced with something else.
But this, for eg, means an Antiochid player who gets into masses of silly wars with remote powers is going to get caught out when that is also where their objectives are.