Working with multiple scenes?

I've never worked with multiple scenes in Unity before -- I always worked with Unity Free back when that was a thing, and I believe loading multiple scenes used to be a Pro-Only feature. Once that model got dropped, it was never something I really played around with at all. With the new multi-scene editing stuff, it's come up on my radar as something I'm curious about looking into, but honestly I'm not sure exactly what would be the best way to use it.

What is the best way to take advantage of multiple scenes if, say, you wanted to create a large world with acceptable framerates? Are there any tips or "rules of thumb" that can be used to determine whether a scene should be broken up into smaller scenes?

I imagine it's probably good practice to make all building interiors their own scenes so they can be loaded or unloaded as needed, as well as any sorts of villages and the like that are "far away" from the player. What about something like a large 2 KM^2 terrain or forest?

Can we "instantiate" scenes in different locations? For example, say we compartmentalize a village scene, which has lots of AI and houses and stuff. We want to use this village scene in a couple different places. Let's assume for simplicity's sake that we're okay with the geometry of the scene being the same, but we want the AI in the scene to act independently of each village instance (i.e. if 2 villages are next to each other, the AI shouldn't be a mirror image of one another).

Additionally, we want the AI to move around and pathfind on the terrain of the "parent" level, while still "belonging" to the village scene instance so they can be unloaded once the player leaves the area (let's assume we're also okay with the AI "resetting" to their defaults once the scene gets loaded again). Would they be okay pathfinding on something that doesn't "belong" to them? I don't see why not, but again I've never worked with multiple scenes before so I'm not sure about the details.

Thanks!