Do remarketing custom audiences keep growing by themselves as there are new potential superfans clicking but not opting in, or do we have to constantly update it by creating a new one?

If you have someone visiting your website or landing page and did not opt in, wherever your pixel is, it is constantly collecting the data of what your fans did or did not do something. You can customize it to track for the last 7 days, or you can choose up to 180 days. It's going to keep track of people for the last 180 days. That audience will always be refreshing. There are other types of audiences (that are not pixel related) that do need to be updated, like video-view audiences.