Apple - Where does App Store download files to?
When finished, it puts them in the Applications folder.
OS installers are all called "Install OS X [OS Name]" which might not be what you expected, if you're looking alphabetically for instance for El Capitan.
Prior to completion they are squirrelled away deep inside /private/var/folders
& not easy at all to find.
For me it was: /private/var/folders/yy/v7l5q9l962j23_n6ttp9cxw00000gn/C/com.apple.appstore/1127487414/iyu1194865269351609080.pkg
To find it out for yourself, open "Activity Monitor" -> Select the process "storedownloadd" -> Open files and Ports -> then you should see the path.
Since the location is generated in a different spot, you have to crawl the filesystem to locate the folder(s) that contain these files.
cd /private/var/folders
sudo find . -name "com.apple.appstore"
# This will throw some errors due to inaccessible directories, so it may be
# better to use
sudo find /private/var/folders -name "com.apple.appstore" 2>/dev/null
# or, making use of the fact that the result will be within TMPDIR
ls -d $TMPDIR../C/com.apple.appstore
If you copy the location and type “open “ with a space and paste the copied location, you should see that folder in finder.