On Android, when you use geckodriver with Selenium to control Firefox for Android (Fenix), the Marionette server is started automatically by the Firefox for Android (Fenix) app itself.
geckodriver acts as a proxy between Selenium and Firefox.geckodriver launches Firefox for Android, it passes the --marionette flag (or equivalent) to the Firefox process.geckodriver: The server starts only when Firefox is launched with the correct flags by geckodriver.So, the Marionette server is started by the Firefox for Android app itself, not by geckodriver or Selenium directly.