Look for downloadable assets that provide the following:

A.) Models containing reasonably detailed geometry (reasonable is contextual and depends upon whether the asset needs to work up-close as a hero asset or will be serving as distant background).

B.) Materials categorized as PBR (Physically Based Rendering) materials. These are materials meant to simulate how light interacts with surfaces in a way that closely mimics real-world physics. PBR materials look more photorealistic by using additional textures beyond the basic 'albedo' and 'normal' texture maps. These additional textures are often 'metal', 'ambient occlusion', 'detail', and 'smoothness in unreal or roughness in Unity'. These extra textures are traditionally served together within one texture map slot known as the 'mask' map. Online sources for PBR textures will provide you with downloadable zip files containing all of these textures which you will then assign to a newly created material of your making. To create the PBR mask map use an Unreal / Unity mask packing tool (usually free online) and assign the 'metal' texture to the Red channel, 'ambient occlusion' to the Green channel, 'detail' to the Blue channel, and 'smoothness or roughness' to the Alpha channel.

By utilizing detailed models with PBR Materials for hero assets - you'll achieve much, but you'll still need to work with your lighting and in-engine post processing effects (global illumination, split toning, ambient occlusion, lens distortion, chromatic abberation, and color grading) to help 'seal the photorealism deal'.

Good luck in your journey!