It appears to me that the item best in focus is the bark of the tree between the two red vehicles. With a long telephoto lens, you will generally need to place a focus point on the area of interest in a crowded scene rather than letting the camera "guess".
The depth of field in focus is limited with a telephoto lens even at f11. I am guessing your camera was around 200 feet from the tree that is in decent focus. With an 800mm f11 lens at 150 feet, the total depth of field in focus is around 12 feet so much of what is in the photo will be out of focus. If you are further away, then DoF increases but it is still pretty shallow compared to more normal focal length such as a 200mm lens.
The EXIF data from the image shows it was captured at ISO 10,000 and significant noise reduction takes place at that ISO which will further reduce detail. Some of the AI noise reduction tools do a pretty good job of estimating what is noise and will fill in what the algorithm thinks is the correct missing data but they aren't perfect. With traditional processing, detail is reduced to smooth noise and that is going on to some extent with your image. When noise reduction is applied, severe cropping makes the results far more apparent. If you can frame the capture where the desired image pretty much fills the sensor then you can get away with a far higher ISO than if you need to crop and magnify from the sensor.
Attached photo was captured with an EF 800 f5.6 at around 100 feet and you can see that the grass just inches beyond the target bird is beyond the depth of critical focus. The f11 wide open won't be this shallow but you still won't have a lot of depth of field. Captured with EOS 1DX III and EF 800 f5.6 @ f5.6, 1/800, ISO 1250.
Rodger
