This may have changed in the last decade but I remember while the Common App had mostly the same basic fields the College's using it could still attach several custom supplement applications to it (eg essays). So it's not a truly common system.

In talking to the parents of some students applying this year to colleges, the common application is truly one common app, including the same essay.

The consequences of this is now it is truly much easier to apply to a bunch of schools for undergrad.

Note, this is not the same for grad schools. I have been applying to graduate schools, and they seem to have their own systems (but there seems to be a common platform called "Applyweb").

I've written between three and ten custom answers for each college to which I have applied. For the schools with more, there are usually three extra essays plus a bunch of short-answer questions. I've also applied to a good few honors programs, which have about three additional essays apiece.

The only thing the common app does is standardize the name/dob/address form that's two pages of paper anyway. The essays, which are by far most of the work, are mostly different. Oh, and the schools all still charge full application fees.

It makes sense that the essays might be different since the schools are different and are looking for different things.

There is one shared essay, but back when I was applying (which wasn’t all that long ago) I had to write additional essays per-college in their college-specific question sections.

It has not changed.