An app can be in any directory. It is suggested to move them to the Application folder so they are easy to find.
When you downloaded Slack it probably went to your download folder. Did you copy it from there to Applications? If so follow directions below. If not, it could still be in your download folder.
There are 2 Application folders. One is under your home directory and the other is at the root level. Slack should in the Application folder at the root level. From a finder window you can right click on the directory name at the top of the finder window. Click on Macintosh HD and that will display contents of the root directory. You should find the Applications listed there.
From the terminal, cd to the root level and do an ls to see the other directories (folders).