The Unity Splash Screen is uniform across all platforms. It displays promptly while the first Scene loads asynchronously in the background. This is different from your own introductory screens or animations, which can take time to appear due to Unity having to load the entire engine and first Scene before displaying them.
The Unity Editor allows you to configure a Splash Screen for your project. The level to which you can customize the Unity Splash Screen depends on your Unity subscription. Depending on whether you have Unity Personal or Unity Pro, you can disable the Unity Splash Screen entirely, disable the Unity logo, and add your own logos, among other options.
For more information about configuring the Splash Screen, see our Documentation page.