athyg,
Just so you understand: To make any edits you will need both the After Effects project file - that's the .aep file AND and source material that was used as assets within that. For example: The final video might be a combination of original footage, images, audio, graphics and other material. Those items are not stored (embedded) within the .aep, but are linked external files. You will need both the .aep and the source files to edit the project and output a new final .mp4 video.
The only exception to this is that certain .aep files might just be constructed from elements that can be created totally within Ater Effects intself. A simple text animation is one example. In those cases there might be no external source material involved, but that is the exception, not the general situation.
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