Where in this code does the switch to 32bit protected mode happen

I read about how to switch from default real mode to protected mode and I wonder where the switch in my code happens. Theres one part that I copied and dont fully understand, which is this:

global loader
global stack_ptr

extern main

MODULEALIGN equ 1<<0
MEMINFO equ 1<<1
FLAGS equ MODULEALIGN | MEMINFO
MAGIC equ 0x1BADB002
CHECKSUM equ -(MAGIC + FLAGS)

section .mbheader
align 4
MultiBootHeader:
  dd MAGIC
  dd FLAGS
  dd CHECKSUM

section .text

STACKSIZE equ 0x4000

loader:
  mov esp, stack+STACKSIZE
  push eax
  push ebx

  call main

  cli ;clear interrupt flag

hang:
  hlt
  jmp hang

section .bss
align 4
stack:
  resb STACKSIZE
stack_ptr:

Since I can use the 32bit registers normally and using interupts fails I guess I am in protected mode, but I dont understand where the switch happened. And I read when using protected mode you have to set up memory paging, however is this just recommended or mandatory?