It is a DE, like regular KDE, GNOME etc. At your OS’s login shell, you should be able to select it once installed. If it is the only desktop environment on that system, it will not even be needed to get selected as the appropriate session gets started when you login.

Arch (and derivative) systems can install it through the AUR, but I do not know any distribution that offers it through their normal distribution channels (packages sources) already.

Did you tap the text “Battery Information”?

Pixel models <=5 have Qualcomm SoCs in them, while model >=6 has Tensor SoCs developed by Google (but fabricated by Samsung IIRC). No Samsung Exynos SoCs in them.