How logs will come to UART serial console?

Hi , Here is my question. when we put 'printk's in kernel code the logs will go to the log_buffer.(i observed that from implementation) But when UART is enabled(in the bootloader), the same printk logs are going to the serial console. As per my study printk doesnot implementing anything that writes to the UART console. Now here are my questions, how this is being achieved? Where this is implemented?(which file) Where wil be the parsing of the cmdline string sent from the bootloader is done? Thanking you, sandeep kumar.