Part 2 is about the evolution of it.
To attach "flaps" to my sketchbook pages I use washi tape and regular cello tape of various sizes (3/4 "Scotch Magic Tape is used the most). The trick is to apply the tape on both sides of the paper - then the connection is stronger and more suitable for working with it (opening and closing).
Washi tape sometimes is used to create a frame for a sketch. And post-it tape is used to hold some pages that keep falling out of order (it is much less sticky than washi tape and can be re-applied).
I use UHU glue sticks, PVA glue (often Elmer's), and Matte Acrylic Medium (by Utrecht) to glue things to the pages of my sketchbooks if I just lay them on.
Page protectors are mostly any transparent paper that I can get my hands on. I used glassine paper (harder to draw on because of the coating), I used tracing paper of all sorts, right now I used mostly vellum paper.
I use scissors and exacto knives to make interesting shapes and folds out of the sketches - t makes for an interesting flow when I am combining a lot of different things on the same page. And I also enjoy sketching with scissors - those sketches often are a reason to add more layers to my sketchbook too.
Here is a video with some examples: