Warning! If you are motion sensitive and on macOS, have someone else do your upgrade to Sonoma. The initial launch screen is a motion animation behind the password field. I haven't tried re-launching so don't know whether this is a one-time thing or an every-time thing on power up. #accessibility
It's apparently automatically set on upgrade. Apple Accessibility Support walked me through disabling it, but it was their first time with this case and things may change. 1) For Screen Saver, choose one of the Other ones. 2) On the same settings pane, turn off Show on All Spaces 3) For Wallpaper, if it's not your preferred wallpaper then choose that or anything that doesn't have an icon on it.
Just so you know, they'll need their own account on the Mac, since the default boot is to that user once the install is done and they'll need the password to the account they used so they can log back in and get rid of the animation on the login screen.