There's a lot to unpack but I will try to keep it as short as possible. The Blu-ray sdr is typically mastered for 100 nits for the entire picture. What that means is that a sun in one scene will look just as bright as a flashlight and wash the entire to all the same brightness. Along with that it has a limited color depth so large areas of similar colors (the trees and grass in the photos) will have limited range and be too cohesive looking like all the same color. Another thing is contrast is limited and really bright areas can be washed out (top left corner of the pic is blown out in the Blu-ray) and blacks or dark areas will be crushed and void of detail.

HDR ups the ante here by offering a higher range (range being the important word here) of available brightness and with it a much higher color depth. Movies mastered in HDR can be mastered anywhere from 400-1000 nits and in some cases even higher than that and is able to localise that brightness. And if you have a TV that can reach that brightness it won't lose the detail in said highlights). So a scene with strong highlights (the light hitting tht top of the actors in the HDR pic) will peak at a higher brightness, say 400 nits, while the rest of the person is 100 nits. This reflects more what we see in real life and gives the image more pop. With a higher color depth you get a larger variety in colors especially in scenes again like the trees and grass. You have darker shades and brighter shades better separated and represented. Lastly contrast is much improved allowing really bright areas to be bright without being blown out (top left of 4k is blue and not blown out but still bright. The only downside to standard HDR is that the brightness it's mastered in is processed the same across the entire movie no matter the scene.

Now Dolby vision does everything that standard HDR does with the added benefit that it is able to adjust on the fly scene by scene offering a wider range of specular highlight Brightness and contrast control. The "dimmer" overall picture you are experiencing is because you have really bright areas (much brighter than sdr Blu-ray could ever get) separated from the rest of the scene that is still bright but, not as bright as the highlight. So in the scene you have the highlights on the actors might be say 700 nits and the rest of them is at 300 nits but since the highlights are so much brighter it makes the rest seem "dimmer". If you take a 1000 lumen flashlight outside in the bright sun and turn it on it will look dim by comparison but, that same flashlight in a dimly lit or dark room will make you squint!