I am wondering how on earth people make thumbnails!

Usually, they make a sprite designed to hide when the project starts using code similar to this:

Then, the project is only saved when the thumbnail is visible.

DinoGirlLogan wrote:

(#1)
I am wondering how on earth people make thumbnails!

do you mean they design the thumbnail or they allow the thumbnail to be shown?

if its the first, they probably just design it using the paint editor, since anyways they are probably gonna use assets in the project itself in the thumbnail, or if they're feeling professional they'll use some other image editor app.

if its the second, you can use the following code for the sprite that contains the thumbnail:

usually scratch shows the last state of the stage as the thumbnail of the project, so before leaving the project, show the thumbnail such that the thumbnail is visible

If you're talking about how people make really good thumbnail art. I would blur the background a bit, then have the main element in front, with the text, the text i use outline. and move it down a couple of times to make it look cool.

(I had to reword my post a bit cuz filter)

The thumbnail of the project is when the project was last saved. So you can make a thumbnail design with a sprite or background (sprite is used more because you can send it to the front layer and show it when the project is stopped) and save the project with the thumbnail shown.