how did some freshwater fish like sturgeon end up in far-apart places (like the Great Lakes and Caspian Sea, for sturgeon) with no fresh-water path between them?
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Most likely transplanted by people from one body of water to another.
However the natural way that happens (non-human intervention) has to do with certain species of fertilized fish eggs are immune to being digested and when eaten by migratory birds, the eggs are present in their poop and if they happen to use the bathroom in or near enough to the water, the fish are born in the new environment.
There used to be water paths between them. The oceans were higher. The Midwest US used to be under a warm shallow sea, it’s all limestone here. To my knowledge, the Great Lakes are what’s left, but now fresh water.
At one point, big sheets of ice covered the Earth. These sheets melted and created paths that let fish move between far-apart places. These paths let fish like sturgeon travel from the Caspian Sea to the Great Lakes. Basically, It’s like nature made secret underwater roads for them.
Specifically with sturgeon there are species capable of surviving in saltwater. They are also a very old class of fish, so old that they would have been around just before pangea split up. The likely could have bridged those short saltwater gaps easily. They have speciated since in different regions.
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