How to put our default logo instead of unity loading bar and logo in the start.

Pay for Unity Pro

https://docs.unity3d.com/Manual/class-PlayerSettingsSplashScreen.html

The docs (https://docs.unity3d.com/Manual/class-PlayerSettingsSplashScreen.html) say:

    the full customization of the splash screen is only 
    available to those on Plus, Pro, or Enterprise Unity plans

Once you're on a paid plan, you can use the https://docs.unity3d.com/ScriptReference/PlayerSettings.SplashScreen.html API or edit the splash screen from Edit > Project Settings > Player > Splash Image