Looking in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/init/main.c?h=v6.5-rc5 there's a function setup_command_line that seems to set up the built-in command line which is called after setup_boot_config

ok idk what that all was. Here's something more interesting:

In arch/x86/kernel/setup.c it says /* append boot loader cmdline to builtin */. I think that suggests that the builtin comes first. And I assume that the code that queries the command line scans left to right and selects the first instance of an option because there doesn't seem to be anywhere that "loads" args into some kind of structure.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/setup.c?h=v6.5-rc5#n972

#ifdef CONFIG_CMDLINE_BOOL
#ifdef CONFIG_CMDLINE_OVERRIDE
	strscpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
#else
	if (builtin_cmdline[0]) {
		/* append boot loader cmdline to builtin */
		strlcat(builtin_cmdline, " ", COMMAND_LINE_SIZE);
		strlcat(builtin_cmdline, boot_command_line, COMMAND_LINE_SIZE);
		strscpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
	}
#endif
#endif

I guess the best thing to do would be to run linux in QEMU with the EFI system that's provided by a third party thing and test it out.