<Kotlin Singleton Class vs Normal Class> I am new ...

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Kotlin Singleton Class vs Normal Class
I am new in kotlin and i read lots of documentation about singleton pattern and kotlin object declaration. I have some problems about singletons.
Firstly, If singletons are beneficial for device memmory, Why is it used so little?
How can we decide, we need to use singleton or not ?
Why we need multiple instances exactly if singleton beneficial for memmory?
If we don't need constructor can we use object all the times instead of class ?
And lastly i have a class which is authenticating phone...