Log input is handled by the standard log crate. Log output is written by the uefi::helpers module to UEFI‘s stdout handle. On most machines this will appear on the screen, but there are exceptions (such as Macbooks) where the logs won’t be printed anywhere.
The default log level is set to Warn, meaning that only warnings and errors will be logged. With a normal successful boot all logs should be at the info level, so nothing will be printed to the screen.
To alter the log level at runtime, create an empty file called crdyboot_verbose in the same directory as the bootloader (/efi/boot).