store.steampowered[.]com/news/app/1675200/view/3675538825747583355
store.steampowered[.]com/points/shop/c/steamdeck/cluster/1
The 1st link is to Steam's announcement of a sale on their Steam Deck & their Spring Sale, but buried in the print is this: These startup movies are available in the Steam Points Shop – you can check them all out here... You'll be able to set your startup movie in Settings > Customization, and if you’re already using your own custom startup movies (in steamui/overrides/movies/), they'll show up in the new UI as well. If you're like us and can't pick just one, we've added a shuffle toggle to randomly pick one on boot. Even better, these movies can be used to customize your Big Picture startup movie on Desktop too.
The 2nd link is to the available movies in the Points Store.