Preliminary composition

If you want to group multiple layers that are already in a composition, you can perform a pre-composition of those layers. In a pre-composition, layers are placed in a new composition that replaces the layers in the original composition. The new nested composition becomes the source of one layer in the original composition. The new composition is displayed in the Project panel and has a rendering function for it, and you can use it in any other composition. You can nest compositions within each other, similar to adding a video element to a composition. A pre-composition of a single layer can be used to apply transform properties to a layer and change the rendering order of the composition elements.

Nesting is the incorporation of one composition into another. A nested composition is displayed as a layer in the composition where it is contained.

A nested composition is sometimes called a pre-composition, and sometimes the abbreviations precomp. or precomp. are used for it. When a pre-composition is used as an element of the original video sequence for a layer, that layer is called a pre-composition layer.

When a pre-composition is used, layers are placed in a new composition (sometimes called a pre-composition) that replaces the layers in the original composition. You can use a single layer pre-composition to apply transform properties to a layer and change the order in which the elements of the composition are rendered.

Select the layers in the timeline panel and choose Layer > Pre-Composition or press CTRL+SHIFT+C (Windows) or COMMAND+SHIFT+C (Mac OS).
Choose one of the following options.
Leave all attributes in.
Preserves all the properties and keyframes of the layer that passed the pre-composition in the original composition applied to the new layer representing the pre-composition. The frame size of the new composition matches the size of the selected layer. This option is not available if more than one layer, text layer, or shape layer is selected.

Move all attributes into the new composition
Moves the properties and keyframes of pre-composition layers one level further from the root composition in the composition hierarchy. When this option is used, changes applied to layer properties are retained in individual layers within the tentative composition. The frame size of the new composition matches the frame size of the original composition.

Effects can include masks and effects of other layers
Effects that use layers as input, such as “Adjust Substrate” and “Offset Map,” target the masks and effects of the input layer. These layers can be used without pre-composition, so that they can be referenced by the effect.

The control is similar to the “View” menu function at the bottom of the Layer view panel, which lets you render a layer from different positions in the rendering order: from its source, from its masks, or from its individual effects.
For effects with layer properties, open the “Input Parameter” menu to the right of the layer selection and select the target input layer, e.g:

Source. Masks and effects are ignored.
Masks. is directed to the layer after which the masks are applied. Effects are ignored.
Effects and Masks. is directed to the layer after which the masks and effects are applied.

Opening and Moving Between Nested Compositions
Nested compositions are sometimes considered to be upstream of the compositions that contain them, and included compositions are downstream of the nested compositions they contain. The root composition is located downstream of everything; the nested composition is located upstream of everything. The composition flow path is a chain of compositions that are connected to each other by inclusion or nesting. A net of compositions is the entire set of compositions that are connected to each other by nesting.

You can open a nested song (pre-composition) in After Effects in several ways.

Double-clicking a song entry in the Project panel.
Double-clicking a pre-composition layer in the Timeline panel. Double-clicking while holding down the ALT (Windows) or OPTION (Mac OS) key opens the pre-composition layer as a layer in the Layers panel.
Note.
When you double-click a precomposition layer while the paint tool or rotoscoping brush is active, the layer opens in the Layer panel.

To open the last active composition in the same composition network as the current active composition, press SHIFT+ESC.

Use the song navigator.

Use the graphical mini-presentation of the song.