Working out makes it harder to lose muscle because exercise kinda tells your body that its muscles and strength is needed since they are being used on a regular basis.
Same thing with pure strength. Strength is rather a neurological feature than a musclular one. That means practising is often and regularly improves your strength on a given move. With strength trqining you are actually enforcing the neurological connections between your brain and your muscles. It is called kong term potentiation.
So if you want to trqin your soleley your muscles, you should use reps in the 6 to15 range. And really exhausting your muscles like goint all out until failure in the last set.
For strength you should do only 1 to 3 reps never go to failure, always keep your muscles and mind fresh. But you will have to do multiple sets per exercise like 5 sets of 3 reps