Not sure how eSIM works since I use a pSIM on Visible but since the iPhone 12 allows for eSIM you should be able to transfer the eSIM from the 16 to the 12 using the app on the 12. Again, I am unsure how eSIM transfer works much less how iPhone's in general work as I am an Android user.
If you need help try this link. https://www.visible.com/unbox/existing-member/esim
