Yes, MacOS is closed source. That doesn't necessarily mean that anything you do on top it doesn't change anything.
"No, Apple still controls the boot process and, for example, the firmware that runs on the Secure Enclave Processor. However, no modern device is “fully open” - no usable computer exists today with completely open software and hardware (as much as some companies want to market themselves as such). What ends up changing is where you draw the line between closed parts and open parts."