The only way to access your data is by sharing the Apple ID or someone knowing your Apple ID and Password. The first thing to do when you have a doubt that your Apple ID and Password have been compromised, is to change the Apple ID password. Open Settings ➡️ Apple ID (Your Name at Top) ➡️ Password & Security ➡️ Change Password. This will prevent someone else from using the compromised Apple ID and Password
The other thing to check is open Settings ➡️ Apple ID (Your Name at Top): Scroll down and see whether you see any strange or unrecognized device(s). If you do then remove the device.
One other thing to check is to open Settings ➡️ Apple ID (Your Name at Top) ➡️ Name, Phone Numbers, Email: Here, if you see any strange phone number or an email address then remove it and then change the password of your Apple ID as shown above.
Axel F.