Does updating a product download and reinstall the whole product.

Any time a program gets UpGRADED, A New Major Release, from version 19.x.x to version 20.x.x, the Whole program gets downloaded and installed.

If it just an UpDATE, Same Major release but different point release, from 20.1 to 20.2, then more than likely only the files need to do that UpDATE are downloaded.

But it never clear in the CC desktop app if  what they call an UpDATE is really an UpGRADE. They both say UPDATE.

So you need to Read what that Update really is. An Update to the existing version already installed on your system or an Upgrade to a newer Major release.

Also I suggest you turn off the Auto updating of both the CC desktop app and all other Adobe programs. You do that in the CC desktop apps Preferences.

And stop the CC desktop app from starting with your system. It does not need to be running all the time to use any other Adobe program.

Only start it when and if you want to check for updates.