Where is esp_startup_start_app_common() started on the ESP32 (ESP-IDF)?

esp_startup_start_app_common() is called in start_cpu0_default(), which is located in components/esp_system/startup.c

Note that start_cpu0_default() is a weak alias for start_cpu0().

For version 4.4.1 of the ESP-IDF, this is the link to the location where it is called.