Can members of iCloud see content from all devices being shared?

If you are sharing iCloud storage, your data is private. See the following from Share iCloud+ with your family - Apple Support

When you share an iCloud+ subscription, everyone in the family group uses their own account. That way your photos, documents, and other information stay private, even though you're sharing access to the service. You simply share the features and space available in iCloud+ with your family members, so you only have one plan to manage.

However, there are options for sharing if you want to. Those options are available whether or not you are sharing a storage plan. For example, see: How to use iCloud Shared Photo Library - Apple Support and Share files and folders in iCloud Drive on iCloud.com - Apple Support