to add a fan to your raspberry pi you can wire it directly to your 5v and ground, but to control it based on temperature you need to use a mosfet or a transistor, your gpio pins are not powerful enough to switch on a fan

Mosfets

there are 2 main types of mosfets p channel and n channel

with an p channel mosfet the source is connected to ground, the drain to the load, and it will turn on when a positive voltage is applied to the gate

with an n channel mosfet the source is connected to a positive voltage, and  it will turn on when the voltage on the gate is below the source voltage

in this example we use a n channel mosfet but you can do the same with your p chanel mosfet see the hint downbelow

Connections

connect the mosfet gate to the raspberry pi gpio  pin 21

connect the gound lead from your fan to drain

and connect the mosfet source  to ground

Software

import RPi.GPIO as GPIO
import time
import subprocess

fan=21

GPIO.setmode(GPIO.BCM)
GPIO.setup(fan, GPIO.OUT)

gputemp=subprocess.run(['vcgencmd','measure_temp'],stdout=subprocess.PIPE)

temp=gputemp.stdout.decode('utf-8').split('=')[1].strip('\'C\n').split('.')[0]

print(temp)
while True:
	print(temp)

	gputemp=subprocess.run(['vcgencmd','measure_temp'],stdout=subprocess.PIPE)

	temp=gputemp.stdout.decode('utf-8').split('=')[1].strip('\'C\n').split('.')[0]


	if int(temp) > 40:
		GPIO.output(fan,GPIO.HIGH)
	else:
		GPIO.output(fan,GPIO.LOW)

	time.sleep(0.25)
GPIO.cleanup()