You can change any apps to different apps meaning the keyboard, homescreen/launcher, messaging app. The launcher is a primary UI thing which is different from iOS and is totally customizable by just installing a new app.

So you can change the look of anything that depends on an app, but stuff like the control center, lock screen, volume sliders, connectivity icons, notifications afaict can't be changed.

https://niagaralauncher.com is a cool looking launcher that I used to use.

It's a little confusing but I'll say there's nothing ugly like the stock GOS apps that can't be changed and tha unchangeably UI elements match the Pixel UI.

Here's a comparison which will show both the unchangable stuff like control center, but also the Pixel launcher, which you can swap out.

https://www.youtube.com/watch?v=lwNicPJk4lY

I switched from iPhone and once I installed good looking apps I really prefer the look to iOS because it's a lot faster and smoother.