You don't have to use xrandr to create a custom framebuffer with scaling and/or centering, although it is capable of doing so. You can also use Gamescope (https://wiki.archlinux.org/title/Gamescope), which works on both X11 and Wayland, and with any GPU.

Traditionally it's used to launch a full-screen application, usually a game, but you can launch your window manager through it, if you want your desktop session to use a custom resolution with custom scaling and/or letterboxing.