Two ways copying the shape paths have failed me :)

[Evelyn___]

Attempt 956

1. I draw a bezier path shape layer because I know parametric shapes cannot get their shape paths copied into a mask
2. I make a solid layer then draw a mask
3. I copy the shape path and paste it onto the mask
4. nothing happens :DDD

Attempt 957

1. I changed the heart consisting of three shape layers (circle, circle, and square) into bezier paths
2. I add a solid layer then draw a vertex point (mask)
3. I copy and paste the circle path onto the vertex point (the circle is too small)
4. Regardless, I try to copy and paste the other shape layer paths onto the mask
5. It fails
6. I cry and contemplate the meaning of existance. 
   

[thepixelsmith]

1. Select the shape layer's bezier path & copy it
2. Create your solid or select your target layer
3.  Create a mask on the new layer. Either draw a mask or use Layer > Mask > New Mask (Cmd+Shift+N)
4.  Open the Mask's properties and select the Mask Path.
5. Paste

Basically you need to copy the Path into the Mask Path.

[Evelyn___]

I tried that, but after I copy and paste a red bounding box appears, but the mask doesn't change.

[thepixelsmith · Accepted solution]

This is a tutorial I made. Maybe it will help.

https://youtu.be/2qelIEzKyIc

Assuming you are clicking on everything properly it should work.

As Warren suggests below If you have Illustrator you could try to copy the path there and then paste it back into AE.

[Evelyn___]

BRO thank you so much.I subscribed to your channel immediately. Like you don't understand the mental wreckage that shapes and masks were to me for the last 48 hours. It generated a painful core memory lmao.  Do you know if it can only copy one path? I had three shape layers and thus three different paths. I totally forgot about merge paths and how I could use that to make a cohesive shape with one shape path. Also, do you know why the size of the shape layer seems to shrink once I copy the shape path to the mask path? 

 

[thepixelsmith]

The size is based on the initial path created and doesn't take into account any scale adjustments you have made. 

All the transform properties get adjusted when you copy to the mask path.

I believe you can only copy one shape path to one mask path. So if you had three shapes to convert into masks you would have to run the operation three separate times.

[Byron Cortez]

Un tip. Si al crear un Shape Layer mantienes presionada la tecla Alt, vas a poder controlar los bezieres como si fuera una máscara.

[thepixelsmith]

The reason attempt 957 fails is because you can't copy 2 paths into 1. Each shape path has to be pasted into it's own Mask path.

[Warren Heaton]

Try selecting the paths and then copying and pasting them to an Illustrator document.  Then in illustrator use Pathfinder to combine them to one, single path and then copy the single path back to After Effects.