Quite right, most jet engines in about the first half of jet flight needed to be started by external means; hence the term 'start-cart'. It would inject compressed air directly into the engine to start, or directly power an electric motor in the aircraft for such purpose. Then by the late 20th century we got good at batteries and compact gas generators that could be installed in aircraft to start their own engines. This is all similar to the progression from literally cranking the shaft start to electric starters in modern car engines.
