The archinstall script has a list of "profiles" that you can select from (custom, desktop, minimal, server, tailored, xorg).. And if you select "desktop" it will prompt you which DE or WM you want to install. (awesome, bspwm, budgie, cinnamon, cosmic, cutiefish, deepin, enlightment, gnome, hyprland, i3, lxqt, mate, plasma, qtile, sway, xfce4).

By the time you're done with the archinstall script, you basically have a fully functioning arch (ive never used the script seriously, so I have no idea what all remains not set up doing this).

The main difference between Arch and Ubuntu in this regard, is that if you want to run KDE Plasma, you download the common Arch ISO, and select Plasma at installation time. Compared to Ubuntu where you would download the "Kubuntu" spin, so you are selecting Plasma when you acquire the ISO in the first place.

There is no "default" arch DE, so when you install Arch, there is a lot of decisions to make (and you may not know how to make those decisions if its your first distro), whereas Ubuntu makes a lot of decisions for you, so you have to answer no questions to get set up (but you may be set up in a way you weren't expecting). In this regard, Arch really does just feel like building a PC from parts, you just have to pick all the parts. Ubuntu is more like buying a pre-built.