How do I set a video with sound as the startup animation?

Help
1

I know it’s possible on Steam Decks which uses KDE Plasma. So there’s somewhere I’m not looking at well enough.

I want to use this as the startup:

2

I think AI can help you write a startup shell script that will play a video clip in fullscreen mode.

3

I’d like to know if there is a existing feature for it that would work without SteamOS.

4