It's not so much about the assets as the framing, lighting, positioning and blending it all in a compelling way. For instance in the trailer I recognize the large brick building at 0:14 from a cheap/free asset pack, though I don't recall which. And look at it - it's super generic copy/paste layering with some slight variations, and seemingly baked on lighting given the identical shading per floor/layer. But put in a well lit scene with appropriate framing, and suddenly you're here thinking it's a high end custom built asset.

Now a days you can also get basically endless assets for free. Unreal gives away a new pack every few weeks or so on Fab (and has been doing this for years), KitBash3D gives occasionally gives away some amazing assets, and many more. But again none of this matters without some serious artistic and style sense. Given I recognize at least one asset there, I'd imagine a good chunk of his assets are from stuff like this. But you're not going to be able to clone anything comparable even if you had the exact assets he used. Placement and such is way more of an artistic thing than you might expect.