merging 2 Apple IDs'

You can't merge Apple IDs and you can't transfer purchases between IDs. You can try to set up Family Sharing between the IDs to share purchases.

Family Sharing

Family Sharing - Set up

Family Sharing - Use your own Apple ID for Family Sharing

Family Sharing -What types of content can I share?

Family Sharing - If you don‘t see your family‘s shared content

Family Sharing - purchases and payments

Family Sharing - Hide and unhide music, movies, TV shows, audiobooks, and books

If you are asking about Family Sharing, it won't merge purchases into one account. Each account retains the purchases and shares them. Remove an account from Family Sharing, and you lose access to the purchases/updates.

There is no way to merge Apple IDs. Which is why it's a bad idea to ever create a new Apple ID. Instead you should just change the email address for the old Apple ID.

No. As I said, there is no way to merge purchases into one account. You will have to repurchase any content.