If you are referring to the top example screenshot, there are 11 layers not 8. There are 9 grey square layers on a black background layer. The white background layer is on but not visible because the black background layer is on. What you describe is one way of adding images to the template grid but it's not even close to being the best way. In doing it this way, none of the images within the "grey square" boundaries are editable. This is not good!
Two better ways of doing this would be to use clipping masks or smart objects. Either of these ways are COMPLETELY editable.
Clipping mask method.
Grid......
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Layers panel (half) showing grid squares and clipping masked images (which are SO's).
Every image is quickly editable!!!!
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Smart Object method.
With smart objects, you just double click the layer thumbnail and open the PSB. Drag and drop the image, edit/adjust, save the PSB and the image is added or subsequently edited (if needed) to the main grid template PSD file. You can add text, effects, filters, blur, etc. or whatever else you desire.
It's completely editable.
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You can even add two images to one square if you like! Only takes a few seconds. If you don't like it........double click and edit!
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