The device was rooted with Magisk and the Termux app was installed. Every command is intended to be run as root.

ps -A

am help
Activity manager (activity) commands:
  help
      Print this help text.
  start-activity ...
  ...

Not all the activies can be launched from the command line, e.g. the following

am start-activity app.lawnchair/com.android.quickstep.RecentsActivity

causes the Lawnchair app dies with a message on the screen Lawchair has stopped. Fortunately enough the app restarts automatically.

service -h
Usage: service [-h|-?]
       service list
       service check SERVICE
       service call SERVICE CODE ...

Magisk modules are installed into /data/adb/modules/, each module is a subdirectory.