I'm new at ROS and I'm trying to figure out how does the node works with a pyton/C++ code. I have two codes for the same situation: Use a Xbox Controller to move the turtle in turtlesim

FIRST EXAMPLE - Using global variables and a if __name__ == '__main__' with rospy.spin()

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist 
from sensor_msgs.msg import Joy

# FUNCTION - GET JOY VALUES TRANSFORM TO TURTLE MOVE
def joy_to_twist_transform(jData):
    twt = Twist()
    twt.linear.x = 5*jData.axes[1]
    twt.angular.z = 4*jData.axes[0]
    pub.publish(twt)

# FUNCTION - START PROCESS
def process():
    global pub
    pub = rospy.Publisher('turtle1/cmd_vel', Twist, queue_size=10)
    sub = rospy.Subscriber("joy", Joy, joy_to_twist_transform, queue_size=1)
    rospy.init_node('control_2turtle')
    rospy.spin()

# MAIN
if __name__  == '__main__':
    try:
        process()
    except rospy.ROSInterruptException:
        pass

SECOND EXAMPLE - Using class python and a while as a main

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
from sensor_msgs.msg import Joy

# DEFINING CLASS
class pubSub:
    def __init__(self):
        # DEFINE VARIABLE TO KEEP LAST VALUE
        rospy.init_node('c2t')
        self.twt = Twist()
        self.hold = Twist()

    def callback_function(self,joyData):
        self.twt.linear.x = 4*joyData.axes[1]
        self.twt.angular.z = 4*joyData.axes[0]
        self.hold = self.twt

    def assuming_cont(self):
        pub = rospy.Publisher('turtle1/cmd_vel', Twist, queue_size=10)
        rospy.Subscriber("joy", Joy, self.callback_function, queue_size=10)
        pub.publish(self.twt)

ps = pubSub()
while not rospy.is_shutdown():
    ps.assuming_cont()

1 - What I understand about how a node works is: he run the code in a loop until is shutdown. But this does not answer Why can I still save the last value published in a topic (in my head, if I run my code in a loop I start from the beginning everytime)?

2 - Why is necessary to keep creating a rospy.Publisher and rospy.Subscriber in every loop? it is possible to create those guy one time only?(for example using them into __init__ function in the second example)

3 - What is the difference between while not rospy.is_shutdown(): and if __name__ == '__main__' with rospy.spin()?

4 - What is rospy.spin()? in any documentation says that keeps your node running, but the way I see he stuck the code in that line and keep there only.

If any could give any recommendation to read about those problems and improve my understatement of how work ROS works would thanks a lot!