What happens to the iCloud storage that I don’t use each month??

Your iCloud storage plan is a fixed amount (i.e., whatever plan you are currently subscribed to). If does not change each month so there is nothing to roll over. If, for example, you are subscribed to the 50GB plan, then you have 50GB of total storage available for you to use. It is similar to renting physical storage space or renting an apartment -- you pay monthly but the size doesn't change each month and if you don't use part of the space it doesn't affect what happens next month.

You are renting storage. It is like renting a car and even if you don't use it then it will still cost the same every day you have it.