Ask HN: How does iCloud.com work?

I was recently on iCloud, and as a designer who has now got into coding, I was just wondering what technologies were necessary in creating a website like iCloud.com?

I always feel like the website feels like desktop software, and I was just wondering if anyone had any insight into how it's made.

I'm just very interested in how things work, and figured HN would be the best place to ask this type of question.