Hum… If you quit the app explicitly (using Cmd + Q or Quit from the menu), then macOS fully terminates the application. Its processes stop running, memory is freed, and it no longer appears in Activity Monitor.
The issue might be when you close an app’s last window (e.g., clicking the red “X” button); the app typically continues running. This is normal for macOS and follows Apple’s guidelines — closing the last window doesn’t mean the app is done. You can still go to File > New and start working again.
That said, some apps may launch subprocesses that block on system calls. In rare cases, this can prevent the app from exiting properly even after calling Quit.