Duplicate layer and change animation

Hello! I just downloaded Adobe Animate 2021 for a simple animation I need.

I am not an animator, but I need to make this myself as part of a project, so please bare with me. 

I want to create an animation of a poligon in which each side has a triangle folded towards the inside. The animation would consist in all of the triangles moving towards the outside in synchrony. 

I thought this would be simple after I created the first animated triangle. The problem is that I copy-pasted the first layer and when I touch and move the symbol, it just applies to the frame (or group of frames before keyframe). I attach a video:

https://youtu.be/Ue4q8A69Nqk

I think this post might be related, but I could not find the option of precomposing a layer anywhere:

https://community.adobe.com/t5/after-effects-discussions/copy-and-flip-an-animation/m-p/11751134#M159686

Thanks!