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What do you mean by zoom? Scale? If so, you can just select the Scale property from the layer you want, copy and then select all other 30 layers and paste. If there were any keyframes in the original layer's Scale property, they will be pasted from the current time.

From CS3 onwards, you can also copy whole top level property groups (for example, the whole "Transform" property group) and paste it to other layers.

EDIT: I see you edited your question, so I am editing my reply.

To copy the scale of a layer, open the main twirl of that layer and you'll see the Transform properties group. Open the twirl for that, and you'll see Anchor Point, Position, Scale, etc. Click on "Scale" and copy it (Edit > Copy, or Control+C on Windows/Command+C on Mac). Then select the other layers and just paste (Edit > Paste, or Control+C on Win/Command+V on Mac).

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Such a broad question

Anytime you add a new item you imported to the timeline or composition panels, it becomes a new layer.

Anytime you duplicate an existing layer (Edit > Duplicate), you get a new layer.

Anytime you split a layer (Edit > Split) you get a new layer.

Anytime you copy a whole layer (target the name in the timeline panel) and paste it, you get a new layer.

You also create layers when you use the Shape tools, text tools, or create a new solid layer, etc.

This list could go on and on, so instead, I invite you to read the After Effects Help section on Creating layers specifically, and the whole chapter on Layers and properties, in general. If anything isn't clear for you, then let us know.

Enigma: Look at the beginning of my (modified) last reply. I explained in detail how to copy scale (or other transform properties) from one layers to others.