I don't know all the details about their engine, but to be honest, it's not about graphics or details here, but more about the interactivity of all the 3D objects, the possibility to stack a hundred of apples in a single room, leave it here, go to the opposite corner of the map and go back there to find all those apples were you placed them.
Imagine the load on the CPU, it's resource intensive. Which is why it's segmented that way.
It's a unique feature of a Bethesda game, and one of the things that most of the fans want to see return their game, just like we wanted confirmation of this feature for Starfield before its release (which didn't prevent the game from being bad... but that's another topic).
I know there's an Open cities mod for that (does it work for houses interior too ?), and I wonder how they are made, do they retain object placement from the player for every location ? Or do they simplify it ?
Playing Oblivion right now, it's still really good (with all its flaws obviously), and it shows that Starfield just made all the worse game design choices, and it's not because of the loading system. It lacked a cohesive world, planets being random disjointed pieces with procedurally generated maps not linked to each other (immersion breaking), no real ship exploration, no fuel (what a joke), no ship exploration on planet surface... and I could go on as I'm not touching most of the usual Bethesda RPGs issues like level scaling etc...
I'm not too worried about TES VI. It won't suffer from Starfield issues because it will be on a single planet, much easier to do.
But I'll expect them to really work on their AI, level design, dungeon variety, none of the level scaling enemies and loot bullshit (I want zones with high levels enemies, that should be mandatory, and tell the player he should not be there unless he knows what he's doing or wants the challenge), MUCH better combat and animation, no hand-holding markers and quests.
Also the map shouldn't be afraid to have some empty spaces, having a camp, cave or dungeon so close to each other feels unrealistic and made for ADHD people, I want larger maps, with visible and recognisable POI to guide me, rather than relying on a fake compass, or maybe even use constellations/stars. Oblivion make the horse useless as you'll always find 36 POI on your journey to your destination. Not even mentionning that you an teleport to any cities from the beginning of the game (I modded that because it feels so idiotic). You need to feel like you're on a journey to your destination, and good enemy placement and challenge is crucial to that. Of course I'm not talking about Daggerfall giant empty maps here, this shit was dumb af.
Overworld map should also prevents you from going anywhere, I mean having high plateau, cliffs etc... and climbing making a return from Daggerfall and upgradable, as well as levitating.
I love this series, but each subsequent game has been downgraded and casualised and I hope tehy realized that, but I won't get my hopes up too much.
Anyway, Crimson Desert might be destroying every western RPG in terms of exploration and combat if it keeps its promises. But there's a lot of unknown things in every other areas at the moment so we'll see. Too bad it leans more on the action/adventure rather than on the RPG side unfortunately. But it might be the open world Zelda a lot of us wanted since BotW/TokT disappointement.
TLDR : Starfield is shit but there's still hope for TES VI as it's not the same game design. Object placement and interactivity is using lot of resources on the CPU which explains the loadings. Also don't sleep on Crimson Desert.