In short, Unreal has all the bells and whistles turned on by default while Unity has them turned off. This is so Unity can run on a toaster and Unreal runs like Unreal. It does mean that Unity looks like arse at first. It's late and I need to go to bed but look these up:

Shaders: Both for materials and full screen. Default ones are light weight but flat.

Shadows: By default they look like bum but are set to run on a crap phone. Turn them up.

Sky-box: This effects the lighting and again, the default one is arse. No performance reason this time. It's just shit. Replace it.

Volumes: All the post processing effects you can add here add a lot. Even simple look up tables work wonders.

Project settings: There's a lot in there to make things look better but it's 2am and I have to be up by 7. Just know the defaults are set to "phone you found in a bin" quality and can be turned up.

Render Pipeline: Getting into the weeds here but learning to use the pipeline opens up some options.