Why do unknown contacts keep showing up on my iPhone?

Start by checking which account holds them. In Contacts, tap Lists and look through the contacts under each account, such as iCloud or a mail account. If you find the unknown entries grouped under one account, that gives you a place to clean them up.

Turning off contact syncing for that account can remove its contacts from the phone, but it doesn't necessarily delete them from the account itself. Check for contacts you want to keep before changing anything.

They could be sample contacts from a demonstration device, especially if the details look made up. That's only a possibility though. Fake-looking entries alone don't tell you how they got into your account.

Another possibility is that contacts were already on the original phone before you signed in, particularly if it wasn't erased before setup. Since they keep appearing on newer devices, check the synced accounts rather than assuming the current phone is adding them.

It may be worth checking your Apple Account for devices you don't recognize. Open Settings, tap your name, and review the device list near the bottom. If anything looks unfamiliar, remove it and change your password. Unknown contacts by themselves don't prove someone has accessed your account.