Apple has made it possible to move purchases between two Apple IDs for the first time, allowing users to consolidate their apps, music, movies, books, and other purchases under one account. This feature is designed for a single user to simplify their purchases.
To migrate purchases, both accounts must be logged in on the same iPhone or iPad. The primary account (the one moving purchases to) must be logged into iCloud, while the secondary account (the one moving from) must be logged into Media & Purchases settings. The two accounts cannot belong to different Family Sharing groups.
To start the process, open the Settings app and tap on your name, then Media & Purchases. Sign in with the secondary account if prompted, and follow the on-screen instructions to complete the migration. Once done, sign out of the secondary account and log back into the primary account.
The feature is intended for users with multiple accounts to consolidate their purchases under one Apple ID. It’s now available in some regions, but not yet in the UK, European Union, or India.
Source: https://www.techradar.com/phones/iphone/finally-you-can-now-move-your-iphone-and-ipad-purchases-to-the-same-account-heres-how-to-do-it