projection

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This particular resource about projection matrices(https://www.scratchapixel.com/lessons/3 ... know-first) tells us:
that while projection, the points within the view frustum are mapped to the range [-1,1](later converted to [0,1]). But, also the z coordinates are mapped to [0,1], hence the view frustum is converted to a unit view volume.

But, when we project the z coordinates onto a plane at z' , shouldn't all the z coordinates be z', therefore resulting in a plane? What am I not understanding in this projection mechanism?