Conceptually, you can think of a signed exchange as a 301 redirect to a new URL which has already been cached by the browser (so there is no 2nd network event). The cache was populated by the contents of the signed exchange, assuming the signature validates.
There is no "reaching into the document" from the previous click or anything weird like that.
I just wanted to say a quick thank you for going out of your way to answer to many questions here. Despite the obviously hostile environment (I have some reservations about AMP myself), your answers have been very clear, informative and level-headed, so thanks for that.
Conceptually, you can think of a signed exchange as a 301 redirect to a new URL which has already been cached by the browser (so there is no 2nd network event). The cache was populated by the contents of the signed exchange, assuming the signature validates.
There is no "reaching into the document" from the previous click or anything weird like that.