My icloud backup is on. Hope do i view messages i deleted from my iphone 12

If you are syncing messages with iCloud, and you delete them, they cannot be recovered. iCloud is a syncing service, not off device storage.

If you include messages in your backups rather than syncing them with iCloud, you can restore a backup assuming that the messages are in the backup. When you restore a backup, you overwrite the current contents of your device.

This explains how to restore a backup.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Delete a message or conversation

When you delete a message or conversation, you can't get it back. So make sure that you save important information from your messages.