I received a coupon link, but I want to see all the files in my library, how can I add them? All I can see is download.

MY LIBRARY

See the button that says "Add asset pack to collection"? You can't see individual files in the library, because you don'town individual files. You own entire projects.

When the bundle expires after 1 hours, will I lost them again, and have to buy it?

Yes, have to buy again. I tried it.

Do I understand correctly that the coupon gives the right to access all files at once, as soon as you add one of the assets, or do you need to add each asset separately in the provided package? (separately download it so that it appears in the library)

I downloaded them, can't see them on my library!