iMac and iPhone Mail and Calendar don't sync

My signins are identical.

Then admin sign-ins should not affect syncing. Are all devices on the same Apple Account/Apple ID? That does affect syncing across devices.

Do you have the appropriate boxes (Mail & Calendar) checked in the iMac and iPhone's iCloud settings on each device?? If the answer is yes please restart the iMac in Safe Mode then restart normally.

Finally are ANY of the following types of third party apps installed on any of the device:

• Antivirus
• Cleaning
• Security
• VPN
• Maintenance