And init of that ATtiny85 evn looks like this:

And init of that ATtiny85 (inside wiring.c) evn looks like this:

If you rename wiring.c and restart editor, you get main.cpp:5: undefined reference to `init' error, there is that "crappy" heart of arduino:

#include <WProgram.h>

int main(void)
{
    init();

    setup();
    
    for (;;)
        loop();
        
    return 0;
}

WProgram.h starts with:

#ifndef WProgram_h
#define WProgram_h

#include <stdlib.h>
#include <string.h>
#include <math.h>

#include <avr/interrupt.h>

#include "core_build_options.h"
#include "core_pins.h"
#include "wiring.h"
...

And init of that ATtiny85 evn looks like this:

void init(void)
{
  // clock calibration stuff
  // recalibrate clock if it was calibrated by bootloader (like micronucleus)
  #if F_CPU != 16500000L
    if (OSCCAL != read_factory_calibration()) {
      // adjust the calibration down from 16.5mhz to 16.0mhz
      if (OSCCAL >= 128) {
        // maybe 8 is better? oh well - only about 0.3% out anyway
        OSCCAL -= 7;
      } else {
        OSCCAL -= 5;
      }
    }
  #endif
    
  // TODO: detect if fuses set to PLL, regular internal oscillator or external and change behaviour in this next section...
  #if F_CPU < 16000000L
    cli();
    CLKPR = 0b10000000;
    #if F_CPU == 8000000L
      CLKPR = 1; // div 2
    #elif F_CPU == 4000000L
      CLKPR = 2 // div 4
    #elif F_CPU == 2000000L
      CLKPR = 3; // div 8
    #elif F_CPU == 1000000L
      CLKPR = 4; // div 16
    #elif F_CPU == 500000L
      CLKPR = 5; // div 32 = 500khz
    #elif F_CPU == 250000L
      CLKPR = 6; // div 64 = 250khz
    #elif F_CPU == 125000L
      CLKPR = 7; // div 128 = 125khz cpu clock
    #else
      #warning "Cannot prescale chip to specified F_CPU speed"
    #endif
  #endif
  
  // this needs to be called before setup() or some functions won't work there
  sei();

  // In case the bootloader left our millis timer in a bad way
  #if defined( HAVE_BOOTLOADER ) && HAVE_BOOTLOADER
    MillisTimer_SetToPowerup();
  #endif

  // Use the Millis Timer for fast PWM
  MillisTimer_SetWaveformGenerationMode( MillisTimer_(Fast_PWM_FF) );

  // Millis timer is always processor clock divided by MillisTimer_Prescale_Value (64)
  MillisTimer_ClockSelect( MillisTimer_Prescale_Index );

  // Enable the overlow interrupt (this is the basic system tic-toc for millis)
  MillisTimer_EnableOverflowInterrupt();

  // Initialize the timer used for Tone
  #if defined( INITIALIZE_SECONDARY_TIMERS ) && INITIALIZE_SECONDARY_TIMERS
    initToneTimerInternal();
  #endif

  // Initialize the ADC
  #if defined( INITIALIZE_ANALOG_TO_DIGITAL_CONVERTER ) && INITIALIZE_ANALOG_TO_DIGITAL_CONVERTER
    ADC_PrescalerSelect( ADC_ARDUINO_PRESCALER );
    ADC_Enable();
  #endif
}