Do all the phones sync to the same iCloud or does each phone sync independently of other phones on my account?

If you are sharing iCloud storage, Nobody will have access to your data and photos on iCloud, if you are using Family Sharing. Your data and photos remain private.

Family Sharing - Apple

Your entire family can share a single iCloud storage plan large enough to store everyone’s photos, videos, and files. Each person’s files and documents remain private, but it’s easy to see how much storage everyone is using. If you need more space, you can upgrade to a larger plan at any time.

If this is not Family Sharing and you are sharing an Apple ID, yes, that gets tangled. Yes, the other person sharing the Apple ID has full access, too. Sharing an Apple ID is not recommended, and can and does cause problems.