Merging Apple Accounts or transferring data is not officially supported, nor can it be done easily or completely.
Apr. 2018 post by ASC helper Winston Churchill on transferring data from one Apple ID to another. https://discussions.apple.com/message/33333380#message33333380
Feb. 2021 tips from Eric Root https://discussions.apple.com/thread/252468781?answerId=254653802022#254653802022
Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs.
Family Sharing - https://support.apple.com/HT201060
Set up Family Sharing - https://support.apple.com/HT201088
If a message says your Apple ID is locked or disabled "for security reasons", click here and read what to do --> https://support.apple.com/HT204106 If you or someone else enters your password, security questions, or other account information incorrectly too many times, your Apple ID automatically locks to protect your security and you can't sign in to any Apple services.
Also note "What to do after [or before] you change your Apple ID [email address] or password". Click here --> https://support.apple.com/HT204071 - "After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services you're using."
Recover your Apple ID when you can’t reset your password - "If you use two-factor authentication and can’t sign in or reset your password, you can regain access after an account recovery waiting period." - https://support.apple.com/HT204921
If you cannot resolve it yourself, try one of these contact methods:
- Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169
- Click here --> https://support.apple.com/choose-country-region/ Select your country, then a product.
- Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232
● Apple may disable accounts for other reasons in accordance with the terms and conditions of various services ( https://www.apple.com/legal/internet-services/ ). Use the contact methods I just provided in the previous section to ask Apple about that.