HI,I WOUL LIKE TO KNOW HOW TO GET FLYING OBJECTS BACKGROUND LIKE THIS ▶[nightcore] ★ Dust Gold - YouTube
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There are several particle effects that come with AE. CC Particle World, CC Particle Systems II, Particle Playground, Foam, and some others.
You would simply adjust the settings to get the particles moving the way you would like then replace whatever the default particle is with a precomp in your timeline that contains the triangle (or whatever you want the particles to be - ideally this is a pretty small composition, like, less than 200x200 pixels).
If you're new to AE, you shouldn't do anything I just talked about, you should go through these resources first: Getting started with After Effects (I know it may look somewhat daunting, but I PROMISE that working with AE will be a lot less frustrating once you spend the time to go through them all. AE is a big program and that set of resources is the fastest free way to get up to speed on how to use it.)