Generic SingletonMonoBehaviour class.

You can use Package Manager or import it directly.

https://github.com/XJINE/Unity_SingletonMonoBehaviour.git?path=Assets/Packages/SingletonMonoBehaviour

Make inheritance & access simply.

public class Sample1 : SingletonMonoBehaviour<Sample1>
{
    public int value = 0;
}

Sample1.Instance.value += 1;