Scheduling algo in OS

I just wanted to ask that on a particular OS ,is the implementation of a scheduling algorithm unique,lets say I am implementing round-robin in my OS ,so will every task within my system get executed on the basis of the time slice decided by the scheduler or only when resources are less then only the threads or processes within the system would be executed depending on the time slice.

Also ,if round-robin algo is used ,then is there a case that SJF or SRTF may be also implemented,so how does then scheduler decides which algo to use for execution of a particular thread or process.

Major confusion is that if every thread is not time sliced ,i.e only when the resources are less then it will time slice each thread,then what is the use of round-robin and actually how is it overriding this fact that the algo should work only during less availability of resources only ,I mean how do we are first able to supress its working by not time slicing each thread and only when resources are less then it will be implemented,so during the phase while it is nt getting implemented that means at that particular time some other algo must be getting executed,so thats what I wanted to ask that are multiple scheduling algos designed for the schdeuler and soes it depend on the activation which will be implemented depending on the need?