According to The Verge, the announcement that iOS users will be able to transfer their chat history to the new Z Fold 3 and Z Flip 3 was made during Wednesday’s Samsung Unpacked Online presentation. It previously was possible to create backups of your chat history, and even restore those backups to a new device, but not across different operating systems.
Transferring your chat history requires a physical transfer over a Lightning to USB-C cable, not over an internet or Bluetooth connection. The Verge also points out that, if you have backups on Android and iOS, you won't be able to combine both histories into one.
If you want to make the transfer, you'll have to overwrite your Android backups with your iOS history instead.