Why I cannot delete contacts from my Samsung Phone
If a contact is stored on your SIM card can be determined easily: SIM card contacts only how two fields: one for the name, the other for the phone number. If a contact has more data it can not be a SIM card contact. You can also check that easily by removing the SIM card and rebooting the device.
Regular contacts are stored in one or more databases that belong to the device's contacts provider app com.android.providers.contacts. Therefore you find the contact data in the directory /data/data/com.android.providers.contacts (only accessible with root permissions).
You you have removed all online accounts your device can sync accounts to the account data is only available locally. For deleting every contact you may simply clear the app data of the contact provider com.android.providers.contacts. It is a system app therefore make sure to enable the &quot;Show system apps&quot; option in Android settings to see this app.