You’ll want to decide on a desktop environment or window manager (or compositor). That’ll be the biggest determining factor of what things will look like. From there, you’ll want to either read the manual or arch wiki on how to customize the different aspects of it.
If you decide you want a tiling window manager, Hyprland is nice since you mentioned you wanted animations. But it’s only recommended on rolling release distros at the moment. It also might not work well with Nvidia.
What kind of “app behaviour” customizations are you wanting to do? That sounds like it would be app-specific. My main form of app customization is to find ways to change the colour scheme (to fit everything else), and also to change the keybindings (I like using vim-like key bindings whenever reasonable)