Hey there, kelly_g_thatsme.
It sounds like you're wondering if someone else (contract holder) can access your personal information. If someone has access to your Apple ID and password, then they could try logging in to see your content that syncs to iCloud like photos, messages, and so on. You will want to enable two-factor authentication and follow the steps in the article below if you believe someone else has access to your Apple ID:
Two-factor authentication requires a verification code from your trusted phone number or device (like your iPhone) when signing into your Apple ID on a new device.
If you mean contract holder, as the person who has the contract/account with the mobile company, they may have access to certain data (numbers who have called you, mobile data usage, so on) as that can be services that go through the carrier. If possible, you can reach out to the carrier to see what access the account holder may have to your data.
Let us know if you have any further questions. Take care.