Thinking about these two cameras, at first glance you might say that the Z8 is tuned for action and the Sony a7R V is a general-purpose camera. However, the comparison is much closer than say, between the Nikon Z7II and the Nikon Z8, because the Sony a7R V has an excellent autofocus system that borrows heavily from the Sony a1 (and even improves upon it in some ways).
Best reviews and videos about the Nikon Z8
Nikon Z8 vs Sony a7r V: Image Quality
In terms of image quality, the Nikon Z8 comes out ahead in dynamic range because of the base ISO of 64, while the Sony a7R V leads in detail thanks to the 63 megapixel sensor and pixel-shift shooting mode. As such, the better camera for maximizing image quality in landscape photography is up in the air, but it probably leans in the direction of the a7R V.