When I design something like this, I always start with a comp that has the highest resolution the client needs—usually 4K. If the editing is going to be done in Premiere Pro, I use that master comp and the Essential Graphics Panel to create some expression-controlled menus and sliders to reposition the elements on the screen, change the size and even the timing, and then create a MOGRT. Then, in Premiere Pro, the video footage is dropped in a sequence with the desired aspect ratio, sized, and edited for the most effective story based on the aspect ratio (it makes a difference in how you cut and frame). The final step is adding the Mogrt to include the graphic elements, and the menus and controls are used to match the graphic position and even the timing of the edit. I do this because editing video in an NLE is always faster than in After Effects unless you need to apply effects and composites in each shot in the project that cannot be added in an NLE like Premiere.
Chat GBT gave you some pretty amateurish advice.
Step 1, adding a null and then copying and pasting, will do no more than Pre-composing except take more time. Neither is as efficient as creating a comp at the highest resolution, rendering the comp, resizing the render to fit the different aspect ratios and then rendering a resized copy because you save render time. However, none of those approaches consider the framing, timing, relative positions, and even timing issues you should consider when you design for different aspect ratios.
If you are going to do all the work in After Effects, I would start with your master comp at the highest resolution, get the project working well and approved if needed by the client, then duplicate the master project in the Project Panel, resize the comp to the other required size, add as many nulls as needed or pre-compose as many layers as necessary to move and scale elements in the design to make the project work as well in the new aspect ratio.
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