Thanks for playing! All of the objects that are part of the popup effect have a script on them. The script adjusts the rotation and position of the object according to the flipping page object, this makes it look like it is "stuck" to the page. It makes it looks like the object is a child of the page without having to do weird hierarchy stuff. It also rotates the object down then back up for the "pop-up" animation.
I've just made the repository public, so feel free to check out the code. Some features (like this one) were a little rushed and might be a little messy 😅