In the original MediaObject() example you gave, the developer is creating multiple media objects for one piece of media. The developer is sending each resolution value listed in the
for resolution in [720, 540, 480] to the PlayVideo function and then in that PlayVideo function he has written code that will get the URL value that corresponds to the resolution value that was sent to the PlayVideo function and then the PlayVideo function returns the URL value for that resolution to the MediaObject() and creates a unique media object for that resolution.
Just understand that Plex player apps handle multiple versions of a video from a channel plugin differently than they handle multiple versions of a video in your local library. And each Plex player app may handle multiple versions of a video from a channel plugin differently. Some will automatically choose the media object whose quality best matches the users' overall online video quality settings and some do not even recognize multiple media objects for channel plugins and will always just choose the first one listed. And I am not sure which Plex player app you are using, but I have never seen one that pops up with a box listing the multiple media objects for a piece of media in a channel (though it could be Samsung TVs or AppleTV).
The Roku app, for example, will by default choose the media object whose quality best reflects your overall online video quality setting, but you can also choose between the various media object versions that are offered for that video. (During video playback, hit the star (*) button on your remote to bring up Video Playback Settings and they are listed under Streams > Version and you can choose a different version)
Plex Web, on the other hand, just chooses the first one listed and doesn't even show the various media object options. When I pointed that out to Plex Web app developers, they didn't even know there could be multiple media objects for a video in a channel plugin.(The three cable icon has nothing to do with multiple media objects for a video in a channel plugin, that just allows you to change your online video quality setting)
If you want to see how that code in your OP works in a channel plugins on a particular Plex player app, two channel plugins that use code very similar to that and offer multiple media objects for a video (with a unique URL for each resolution) are CBS (720p and 320p) and Food Network channel (720p, 540p, and 480p)