There is a focal point for the primary mirror. Every square cm of the primary mirror will focus the same image of the sky at this point. The additive effect of all these "little mirrors" will make a brighter image at the focal point. The larger the mirror the brighter the image.
Conversely, you can subtract lots of these little squares (e.g. where the secondary blocks the light) and it will just cut out a bit more light.
You could drill holes in your primary mirror and it would still work.
They can make really big telescopes using lots of smaller mirrors.
https://en.wikipedia.org/wiki/Segmented_mirror
Regards, Rob