A general overview of what happens before main() (2019)(embeddedartistry.com)

  .data
   .set L_STDOUT,        1
   .set L_SYSCALL_EXIT,  0x2000001
   .set L_SYSCALL_WRITE, 0x2000004
   L_message:
    .ascii "Hello, world!\n"
    .set L_message_len, . - L_message
  
  .text
   .global start
   start:
    # write(STDOUT, message, message_len)
    mov     $L_SYSCALL_WRITE, %rax
    mov     $L_STDOUT,        %rdi
    lea     L_message(%rip),  %rsi
    mov     $L_message_len,   %rdx
    syscall
  
    # exit(0)
    mov     $L_SYSCALL_EXIT, %rax
    mov     $0,              %rdi
    syscall

  $ as -arch x86_64 hello.S -o hello.o
  $ ld -arch x86_64 -o hello -static hello.o
  $ file hello
  hello: Mach-O 64-bit executable x86_64

  $ ./hello
  Hello, world!