kernel.asm
mov [0xe020],dword Vesa12_putpixel24 ; Vesa 1.2

vesa12.inc
mov edi,eax ; x Where does this eax and ebx value come from?
mov eax,ebx ; y

mov esi,boot_fonts
call boot_log

boot_log:

ret ; Normally it keeps the address after this call and provides a return, so how does it process the commands after the ret command?

pusha
.......
........

The ret-command is there to simply disable boot-up messages. Just remove it to enable the function.