Installed application does not show in Applications folder I have installed Slack App on  my Mac OS and it works all fine. However, I do not see that application in Applications folder. I am very new to Mac and may be I am missing something but to my knowledge, IF App does is installed on the machine then it has to be in Applications folder. 

Is there any reason under which it can not be in the Applications folder?
With any running application that shows in the Dock, you can press the command key and click the application's Dock Tile to have a Finder window open at its installed location, highlighting the app.

If you have Finder's Path Bar showing, (at the bottom of the Finder window), then you can see its path in relation to the volume it's installed on, which it typically the Macintosh HD in the /Applications folder.

Without Finder's Path Bar showing you can right-click (control click) on the folder name at the top center of the Finder window to show its path.