No data is shared with family sharing. Photos are not shared unless you manually enable shared photo library. The location can be shared if you manually enable it in family sharing settings. The only time you have to share location is if you're under 13. What is Family Sharing? Everyone's personal files and preferences stay private When you share iCloud+, family members can't see each others’ photos, files, or documents. And when you share other subscriptions like Apple Music, Apple Fitness+, or Apple TV+, each person sees their own preferences and recommendations — not the whole family’s. How to use iCloud Shared Photo Library Share your location with your family The family organizer sets up location sharing in Family Sharing settings. After location sharing is turned on, the organizer's location is automatically shared with everyone in the family group, and any new family members who are added later. Then each family member can choose whether or not to share their location. Share iCloud+ with your family 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.

Yes. You are only sharing the storage for them, not the photo libraries themselves. Other people in the group cannot access iCloud Photo Libraries from other members in the family group. If you don't have iCloud Photo Library turned then, then your photos are not even on iCloud.

Only if you share an AppleID.

How many photos and videos can be stored in a shared album?

A shared album can hold a maximum of 5000 photos and videos combined. When you reach your limit, you must delete some photos or videos before adding new ones.

iCloud Photo Sharing FAQ

My Photo Stream and iCloud Photo Sharing limits

I realize this is not directly answering the original question, but the information seems relevant because if you have > 5k photos/videos then the answer becomes "No, you cannot share all iCloud Photo Library content with a family member".

No

iCloud Photo Sharing is all Apple has to offer to share photos, but it has many draw backs. It is fine to show off the best photos in shared albums and to transfer them easily, but it it is horribly inadequate to share a joint Photos Library between family members. We cannot sort the photos in the shared albums.The number of photos and albums is listed.And worst of all, most metadata will be lost, when we add photos to a shared album and the quality will be reduced,When shared, photos taken with standard point-and-shoot cameras, SLR cameras, or iOS devices have up to 2048 pixels on the long edge. Panoramic photos can be up to 5400 pixels wide.Shared albums cannot be used to transfer photos to a joint family photos library, unless we want to sacrifice image and video quality and metadata.Even with shared iCloud Storage planes, the only supported way to share a Photos Library is still the sneaker drive: Sharing photo libraries among multiple users - Apple Support