How To Give Each User Separate Applications Folder
Solution 1:
It is easy.
If you download an app from the AppStore, the app will be installed in the /Applications folder by default, if you want to hide it from others, just move it to /Users/someone/Applications/ (which is your own or another users' application folder).
If the app is from a DMG file, just put it into your own (or other user's) Applications folder, and do not put it into /Applications folder.
The application would then appear only in someone's Launchpad.
Solution 2:
You can't really manage this. Unix is a multiuser system and apps are usually shared to save space and ease of maintenance.
Apple and apps from the MacOS Store will install in /Applications however only admin users can install or delete them. Thus you can make it so only you can install or delete.
However non Apple apps that are not via app store can be managed in the way you want if they are written correctly (i.e. there are exceptions) If the install process is just copy from a .dmg disk then they can be copied to ~/Applications