Have you heard of the Grub Customizer? It is a third-party application used to customize Grub. The welcome screen (which is part of gdm3) can be customized, but is complicated. So that would be third party.
The Ubuntu Dock seems fine, but for example, coloring it and styling it, would never ever hurt. Remember you can change the top bar using GNOME Extensions and Tweaks.
For the Yaru Theme, I considered in the past making a theme pack with different Yaru colors, but it isn’t worth it. Besides, Yaru is new. Theme updates are comming quickly.
Everything that needs to be improved is working on it. The desktop however is ran by GNOME Shell. And software is really built on gnome-software, but the apps are customized for people who use Ubuntu.