Sharing email account setup across iDevices automatically

To reiterate - should you perform setup of a new device using QuickStart, or through restoring an existing backup - account settings and credentials are automatically copied from one device to another without the need for any user intervention.

Unless you choose to perform a fully manual configuration of an email account, iOS/iPadOS will automatically set the majority of account settings - but not the account credentials.

Mail accounts are individually installed on each device. Mail messages and accounts are not directly synchronised between devices - this being a common attribute of all computers.

You will need to manually add each mail account that you wish to access on each device from which you intend to access them. Perhaps you'll only keep subset of accounts on your portable devices (e.g., iPhone and iPad) - but wish to access all your accounts from your Mac or Windows PC.

If performing a transfer between devices, or setting-up a new Apple device using an existing Apple device, existing Mail account information will be transferred between old and new devices.

Mail accounts are synchronised between client devices via the Mail Provider's mail servers.

Perhaps you have misunderstood...

Credentials saved in your saved passwords (i.e., Settings > Passwords) are shared between all devices that are signed-in to iCloud with the same AppleID.

When configuring your Mail account(s) on your device, you must enter the account username and associated password - which you can retrieve from your saved passwords.

Should you perform setup of a new device using QuickStart, or through restoring an existing backup - account settings are automatically copied from one device to another without the need for user intervention.

Your mail account credentials, when added to your saved passwords (i.e., keychain), are shared between all Apple devices that are signed-in to iCloud with the same AppleID.

When setting-up any services on a new device, whether initially configuring mail or other messaging service, you must enter the account credentials that you wish to use with that service - which may not necessarily be the same for every device that you use.