Two apple IDs for the same email/person

The Apple ID is definitely created with iCloud domain, the first is example1@icloud.com signed in a device the second is example2@me.com signed in another device its created 15 years back see the article About your @icloud.com, @me.com and @mac.com email addresses - Apple Support

If you created one or more email aliases for your account, be aware of the following:

• You can change or delete an email alias, but if you delete an @mac.com or @me.com email alias, you can't add it back to your iCloud account.
• You can't sign in to iCloud.com with an email alias. Sign in with your @icloud.com email address or the Apple ID that you use with your iCloud account.

You can change the Apple ID from second device ( in which Apple ID signed in as example2@me.com to an Apple ID say example1@icloud.com

For that need to sign out Apple ID Sign out of iCloud on your devices - Apple Support from second device with all account credentials its password initially in records and then sign in with another Apple ID Sign in to your Apple Account - Apple Support

So that same Apple ID will be signed in between two devices on the same network , iCloud subscriptions will merge Share iCloud+ with your family - Apple Support

Some iCloud subscriptions have been merged and somenot because the second Apple ID was not syncing with the first device .

I have this exact problem. XYZ and XYZ@mac.com. I accidentally did this way back before I knew much about computers and thought they were the same thing, and obviously they aren't. I desperately want to merge the two.