Hello.
I wanna code those things that are somehow popular on youtube shorts where a ball is trapped inside a shape and it does something every time it does a certain command.

I am making on with Scratch Cat. But I wanna make the Scratch cat have a trail while it glides.
How do I do this?

There's multiple ways on doing this, but there is two ways, with clones, and with pen.
For clones:

// in cat sprite
when gf clicked
forever
...
create clone of [myself v]
end
when I start as a clone
fade evenly into (10) frames ::custom //change 10 to whatever
delete this clone
define fade evenly into (frames) frames 
// DO NOT RUN WITHOUT SCREEN REFRESH
repeat (frames ::input)
change [ghost v] effect by ((100) / (frames ::input)) 

Pen:

//in cat sprite
when gf clicked
forever
...
stamp
end

//in a new sprite named "Curtain", with your background of choice
when gf clicked
erase all
go to [back v] layer  ::looks
set [ghost v] effect to (75) //the higher, the longer the pen trail
hide
forever
stamp

I would use the pen attachment's “stamp” block. You could do something like this:

when green flag clicked
forever
stamp
end

You would need a mouse trail or sprite tail.
Please give credit if use my code in your project.