iPhone XS Max Screen Resolution

I’m enjoying my iPhone XS Max; the screen is amazing.

The tech specs on Apple’s site says the resolution of the screen is 2688 x 1242. But when I go on the YouTube App, I can watch videos at 1440 resolution. Here is an example:

https://youtu.be/WeqxJylzsWA

How is this possible with a resolution of 1242 pixels?