You have installed some custom AppExchange Apps and they appear in your App Menu?
Or you want your internal team to see only 1 relevant app i.e. Sales app or Marketing app?
Good news. You are 4 steps away.
1. Go to Setup and in Quick Find search for Apps.
2. Click on Apps and and on the Edit next to the app you want to Remove/Add to the App Menu.
Scroll down and make the required edits to 'Assign to Profiles'. In the 'Visible' column you can untick checkboxes for user profiles that should not see the app in their app dropdown. Also, you can select a specific App to be default for different profiles.
4. When done, just click 'Save'.
It is a simple quick change but can go a long way.
As a System Administrator, you want to make sure users can access only relevant information to them, so they can fully focus on what is important.
Good luck :)