c – Make a system call from the Assembly

I'm trying to learn the assembly in more detail and I think that C code compilation and analysis of generated assemblies are a good way to start.

Compile a simple printf ("% d n", 5 ") create the following:

                .globl _main ## - Starts the main function
_main: ## @main
##% bb.0:
pushq% rbp
.cfi_def_cfa_offset 16
.cfi_offset% rbp, -16
movq% rsp,% rbp
.cfi_def_cfa_register% rbp
under $ 16,% rsp
leaq L_.str (% rip),% rdi
movl $ 5,% esi
movl $ 0, -4 (% rbp)
movb $ 0,% al
callq _printf
xorl% esi,% esi
movl% eax, -8 (% rbp) ## spill over 4 bytes
movl% esi,% eax
addq $ 16,% rsp
popq% rbp
## - End function
.section __TEXT, __ cstring, cstring_literals
L_.str: ## @ .str
.asciz "% d  n"

Here are my questions:

1- What is the difference between using callq _printf and make a system call with int 0x80; or system call? is _printf to make the same system call behind the scenes? is _printf defined in ?

2- Where can I see the documentation on the use of such calls i.e _printf. The documentation indicating where to place specific values ​​in which registry for this to work, is this called an ABI?

3- Same question as # 2 but on system calls such as int 0x80;. Where can I see a list of these calls and how to use them in assembler?

I know the Linux manual pages but (correct me if I'm wrong), its documentation explains how to use them in C / C ++ code and not Assembly.