In middle school history we were taught that the Romans renamed the Greek gods and basically copied them. That always confused me.
How does something like this happen? Did they have gods named "Saturn" or "Jupiter" and just add stories from the Greek? Did they Just completely redo their own religion? Did someone decide or was it a side effect of a slow cultural merge?
They didn't copy and rename them, which is a common misconception. They equated some of them in some contexts, but that idea of syncretism was common throughout the ancient world.
There are a few truly "shared" ones, but they're less gods of a pantheon and more mythological culture heroes, like Hercules.
u/KiwiHellenist explains it here
To put it briefly, the idea that the Romans simply copied the Greek myths wholesale and changed the names is a misconception. The short version is that the Greeks and Romans were both drawing on a shared Indo-European heritage with deep roots, and the Romans did absorb ideas, stories, and influences from their neighbors and the peoples that the conquered (among whom the Greeks are by far the most notable, but still only one among many), but they adapted these influences to a pantheon that was already native to their cultural millieu.
I don't have the time or resources to give a full answer of my own, but I can certainly refer back to the work of those who already have done so:
This shouldn't, of course, discourage anyone who wants to provide their own answer with more up-to-date information or from a different academic perspective, of course.