Where is the definition of function millis() located used for ESP32

ah there it is. Thank you gfvavlo

yes very good question. hm well one argument is "oldwards" compatibility to Arduino Uno / Mega
Whenever I would like to re-use some code written for ESP on an Arduino this would require re-editing or do you have a clever solution how to make things like

uint64_t currentMicros = esp_timer_get_time();

compile the same way as

uint32_t currentMillis = millis();

?

best regards Stefan