Hi,
on Mobile, Splash Screens are built in, but you need Unity Mobile Pro to allow custom splash screens...
if you want your own splash screen, it's really just a matter of using Unity UI and have a full screen Sprite that you have visible at the beginning of the app ( as setup in editor, so that there is no glitch), and you fade that image away or hide it when you want to show your app content.
Don't overthink this, it's really just an image that takes the whole screen.
Else, can you detail what you expect?
Bye,
Jean