Each tab is a separate page kept loaded in the same window. They all use memory whether you are looking at them or not — which is why forty tabs slow a machine down.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
several books held open at once, fingers in each.
For example
You have forty pages open across the top and the laptop is crawling. Each one is still running, whether you are looking at it or not.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.