When the world generates, you'll see various texts appearing while waiting.
These texts aren't random, the world is actually processing what the text says.

I've seen a video that showcases how the world generation looks like behind the scenes. So that gives you a rough idea how it's creation works: (https://www.youtube.com/watch?v=bF-_AeRRbmU)

As for rendering, while the world is loaded all at once, it will only draw (or render) the blocks when you're close to it. so when you're moving fast, you sometimes see the tiles ahead of you needing some time to appear.