How do I delete these horrid underwater screensavers in Apple TV 4K?

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You can't so far as I'm aware.

You can swipe left/right to move to another one.

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• While an Aerial is playing, you can switch between locations on your Apple TV. Just swipe right or left on the Touch surface of your Apple TV Remote.
• You can't choose a specific Aerial to play on your Apple TV. But you can set how often new Aerials are downloaded. Just go to Settings﹥General﹥Screen Saver and select Download New Video.
• The number of Aerials that you can download depends on how much storage is available on your Apple TV. To manage storage on your Apple TV, go to Settings﹥General﹥Manage Storage.

Aerial screen savers on your Apple TV - Apple Support

The upcoming tvOS 14 is said to bring more user controls to Aerials, by selecting Themes to be On or Off.

• The number of Aerials that you can download depends on how much storage is available on your Apple TV. To manage storage on your Apple TV, go to Settings﹥General﹥Manage Storage.

I was looking at the numbers on my Apple TV's the other day, and now wonder if it doesn't depend on more than how much space is left. I have one with 88 videos on and 10GB of free space and another with 24 videos and 18GB of free space. It might be co-incidence, but those that are used more often tend to have more free space, all my Apple TV's have the same apps on them.

That is pretty neat, that you are able to compare such.

88 is also pretty close to the limit of available clips.