How do I recover missing photos after iPhone 12 mini setup?

I was setting up a new iPhone 12 Mini. I got a notification on my old iPhone saying that I can set it up with my Apple account and I proceeded. Everything worked out fine but after awhile I saw that there were some missing photos. Luckily I still have my old phone with the extra 900 photos and videos that didn’t get transferred. I’m wondering why the images didn’t get transferred and what I can do to bring them onto my new phone.

[Re-Titled by Moderator]