Sync email accounts across devices (MacOS & iOS)

Yes if you use IMAP. All devices accessing that email account will be synced. So if you delete an email from the synced account on any device it will be deleted on all devices. If you create an email on one of the synced devices the email will show up on all synced devices. Accounts will not automatically show up you have to add each email account to a synced device before that account will sync on the device.

No. How would the second and subsequent devices know what email account to sync with until you told it? It is not difficult to set up an email account on each device. The settings on each device are the same so it is a simple task. You only have to do it once then syncing is automatic. Once the account is configured on each device your email is synchronized across all devices.

Use IMAP email protocol to sync email accounts across multiple devices. In the email client on each phone setup the email account to be synced.

Mail settings you might need from your email provider - Apple Support

Mail Server Settings preferences on Mac - Apple Support

No. You add the IMAP account on your Mac and on all other devices accessing the IMAP account. Then email on all devices will be synced among all other devices accessing the IMAP account.