So far there's no way of doing a short video on boot, at least that I know of.
Everything else depends on what you're using. You can set up a boot logo for Hekate, more details are here:
https://github.com/CTCaer/hekate/blob/master/README_BOOTLOGO.md
Atmosphere's fusée-secondary can also use a custom splash screen, check it out here:
https://github.com/Atmosphere-NX/Atmosphere/blob/master/docs/components/fusee/BCT.md
ReiNX has a toolkit for using a custom splash:
https://reinx.guide/guide/post-installation
I used to use SwitchBlade for this a long time ago, but it hasn't been updated in a long time:
https://github.com/StevenMattera/SwitchBlade
I'm sure there are other methods too.