How do I retrieve my past emails from the cloud for backup

The master copy of all email is on the email server, not in any cloud. Except for iCloud email, go to Settings/Mail/Accounts, tap on an account, then check the setting for Mail Days to Sync. iCloud will always sync all mail.

What does iCloud back up? - Apple Support

iCloud backup doesn’t include information that is synced and stored in iCloud, like Contacts, Calendars, Bookmarks, Mail*, Notes, Voice Memos, Messages*, shared photos, iCloud Photos*, Health data*, call history*, and files* you store in iCloud Drive.

*-If you had turned on Messages in iCloud, iCloud Photos, Mail, Health data, call history under iCloud, and files you store in iCloud Drive your content is automatically stored in iCloud and is available on all devices that use the same Apple ID