Download E-Books to Your Tablet or Laptop to Read on Trips to Remote Areas

On a recent trip to the tropics, where I had no access to Wi-Fi, I was surprised nevertheless to find that I was able to read several e-books -- including a couple of massive e-books -- on the Kindle Cloud app that I had placed on my laptop. I was using a Chromebook having no hard drive, and therefore no storage other than on the cloud, and yet I was able to enjoy hours of pleasant reading. I completed, in this manner, Justice Sonia Sotomayor's autobiographical My Beloved World.
I was also able to read e-book content on my tablet in the course of several flights aboard an airline that provided no WiFi service in the air. I accessed that content as easily as if I were using the WiFi in my own home. And I learned a lesson: to download e-books I was eager to read in advance of undertaking any trip.
I am not sure I understand how the e-book content on my laptop and table was acquired in the course of a flight, and in remote and un-developed islands where there was no Wi-Fi. And I'd be grateful if a reader would explain it all to me. Was I "grabbing" that content in the same way tha cell phone would acquire a connection? Had the e-book content been stored in some fashion within my laptop or table (which seems unlikely). How did I "pull it down”"in places where I was bereft of a Wi-Fi connection?
Since I enjoy reading while traveling, and since I am often in areas where I have no standard connection to the Internet's content and news sources, I will henceforth make certain that I have acquired the content of several vital books that I am anxious to read. I will become an e-book addict, knowing that at the very least, I will be able to read that content on the laptop and tablet that now accompany my trips.