it's a migration between two sqlite database schemas (ChatStorage.sqlite -> msgstore.db) once you got the mapping. Here's a public snippet that's still mostly true. A bit of adaption and cleanup is needed for some messages to not crash the receiving App and modify media paths referenced in the target device.

What's in the toolbox to make the migration and rough outline?

• libmobiledevice-utils (get the backup)
• ideviceunback (extract the backup)
• sqlite (snippet migration + delete control- and data/media_url=null messages)
• adb (fetch empty Android database, then push newly migrated database and media)

Forensic firms and academics did writeups years ago and it's packaged in some commercial solutions. I'll get around to post a current snippet based on @paracycle work