The clues for the final puzzle, this meta puzzle, are the first two lines of each epigraph.
The first goal will be to take specific panels from each door to form a new picture.
All the panels you will need will be from the seven doors.
The seven answers from chapters 1 to 7 can fit on the 5x7 grid of each door. Write each answer on each of the seven doors.
The seven doors are composed of a different element. The iron door, for example, has the periodic table element Fe.
If you wrote the answers on the iron door, and the iron door has the symbol Fe, then you will need each panel from the iron door that has either the letter "F" or "E".
This just solidifies you will not need to use a panel from each door more than once when forming the complete picture.
When you assemble your picture, you will need 20 panels from the seven doors. Also, the fact that it says "guide" means the final picture will guide you somehow, like a map.
Each element also has an associated atomic number. You can ignore those. It was only used for Chapter 8. If you were also keeping track of mass, you can ignore those too.
It's a jigsaw puzzle. There should be only one orientation that fits perfectly. You will not need to rotate any piece.
"Dig" refers to the verb "dig" and not short for anything. The "10" in the page icon means to go to page 10. There is an inversed X with a tiny arrow pointing at it. And outlined will be the word Jess, which was from the door saying "Jess alignes with your goals."
Jess aligns with the third Jess on page 10. The circle aligns with the page number circl on page 10.
The X is reversed because it indicates an area. Start on Page 10, and imagine it is in a book. Note where X is and what it highlights. Then 'dig' to the page underneath, which is page 8. What is revealed? Continue until you reach the front of the book.