How do I delete contacts from all my devices at one time

You would need to use the same account such as iCloud, Google, Outlook, etc, on all of your devices to sync your contacts. Then removing a contact card on one, would remove it on all other devices too. Contacts app does not allow to delete multiple contacts at once.

If you want to remove all contacts from the device but not from the account, go to the account settings and toggle off “Contacts”. You would need to do this on each device.

If you want to remove all contacts from the account, access your account via the website and remove it from there. Change made to account will appear on every device where the account is used.