I do not have any camera nor lights yet whenever I switch from LookDev mode with textured models to Render preview it all suddenly gets darker and glossy. I've noticed that adding light to the scene actually reduces darkness according to position. Also in render workspace there are two spheres in right bottom corner where one is completely metallic, other is white but with shadows. How come the scene has these modifiers and how to completely remove them?
Since you don't add any light to your scene, the render will be dark which only light up by environment light. (It will be weird when you can see object in a place with no light at all)
If you are not going to render it, use DevLook will be a better and easier way to look the scene with the power of EEVEE. You can also use viewport render if you want to get a screenshot.
- left one: reflection map
- right one: diffuse map
If you want a shadowless and reflection less render: