How do I access iCloud backup from my old iPhone

I recently bought an iPhone 16 and transferred apps and data from my iPhone 13 via an iTunes backup and restore. I then found that not all the photos and messages like WhatsApp went to new phone. I also ran iCloud backup and sync on both phones (maybe foolishly) then wiped the old phone as it was being sold. When I go into iCloud I can see both phones there and the old one seems to have more data as storage amount is larger. I’m wondering if the missing photos and messages are in the iCloud backup of old phone and if there is a way to get these or merge these onto the new phone that is being sync’d into iCloud.

[Re-Titled by Moderator]