Every time I see this asked, the person asking gets highly-upvoted answers that are complete bullshit. You want a real answer?

YouTube stores all of their video files and audio files separately. It's a clever way of saving server space—you can have all of the different video files (at a bunch of different resolutions and video codecs), and then a few audio files (at different bitrates and formats), and then on the fly pick the best video file for the user, pick the best audio file for the user, and play them back at the same time. For example, here's all the individual files that YouTube generates/stores for the Costa Rica video from your post.

In a browser, YouTube uses Media Source Extensions which allows them to combine those video and audio streams, and do things like let you switch resolution without the video stopping (or even dynamically switch the resolution based on available bandwidth without interrupting the video.)

Safari on iOS doesn't support Media Source Extensions. So in addition to the multitude of individual video-only and audio-only streams YouTube generates, they also generate a few versions that include both the video and audio in one file—but only up to 720p. That's why Safari only shoes those options, it's all it's capable of playing, because there's no assembly needed by the browser. (Interestingly, Media Source Extensions is implemented in Safari on iPadOS, which is why you'll see more resolution options there.)