Intro Video file size - how does Kirby handle it with slow internet?

I would like to understand: when I’m using a longer intro video with 100 Mb file size, how does Kirby handle this when someone opens the site with a slowly internet connection, e.g. on mobile?
In all tests right now it loads immediately and everything is fine also on the iPhone.

Just would like to understand the system and hidden rules…

Kirby only does what you tell it to, there is no hidden magic anywhere, and therefore you are responsible for dealing with slow internet connections.