I only have an IPhone. Are my contacts backed up to the cloud. I notice that I deleted certain numbers but they reappeared. Could it be possible that the iCloud is set up on a different device that doesn’t belong to me? What else could cause this?

How can I tell if someone else is logging into my iCloud account from their device? I only have an iPhone. My contacts keep changing. I’m not changing them. Contacts are not an app in the iCloud account that I see on my iphone. I only see mail, notes, find my phone, and photos. When I go to setting on my phone I see contacts and calendar etc. I’m not sure how this is possible. I want to know how I can see all the devices that are connected to my Apple ID?