Preinstalled applications are in /system/app folder, these are the one that can not be removed without root access. User installed applications are in /data/app but also can be seen nor downloaded without a root access.
If you just want to get an APK file of something you previously installed, you can do this:
Get AirDroid from Google Play, this app let you wirelessly manage, control and access to Android Devices from a web browser Over-The-Air.
Access your phone using AirDroid from your PC web browser,
Go to Apps and select the installed app,
Click the "download" button to download the APK version of this app from your phone.